Handle signals on Windows in exit tests.

[grynspan]

This PR adds support for handling signals as distinct exit conditions on Windows. Previously, we didn't support them because Windows itself has minimal support. However, through the judicious use of "stuffing a bunch of bits into a 32-bit field", we are able to propagate raised signals out of the child process and detect them in the parent process.

We do this by installing our own signal handlers for all signals supported on Windows, then masking the caught signal against an NTSTATUS severity and facility that are unlikely to be reported by the system in practice. In the parent process, we look for exit codes that match these values and extract the signal from them when found. Because the namespace for exit codes on Windows is shared with uncaught VEH/SEH exceptions (which are propagated to the parent process as NTSTATUS codes), there is the potential for conflict with some hypothetical real exception code, but I'm taking a gamble here that my choice of NTSTATUS facility is unique.

(Bonus points for there not being convenient macros to construct an NTSTATUS code anywhere in the Windows SDK I can find.)

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

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[grynspan]

Some additional documentation blobs I'll put here for posterity in case we decide we want to add them:

For .exitCode(_:):

  ///
  /// On Windows, the system uses [Structured Exception Handling](https://learn.microsoft.com/en-us/windows/win32/debug/structured-exception-handling)
  /// to handle some error conditions. If an exception is raised and is not
  /// handled, it will terminate the current process with an exit code equal to
  /// that exception's [exception code](https://learn.microsoft.com/en-us/windows/win32/debug/getexceptioncode).

For .signal(_:):

  ///
  /// ## Signal handling on Windows
  ///
  /// On Windows, by default, the C runtime will terminate a process with exit
  /// code `3` if a raised signal is not handled, exactly as if `exit(3)` were
  /// called.
  ///
  /// In order to accurately report the exit condition of an exit test,
  /// the testing library installs signal handlers for all signals supported by
  /// Windows before an exit test runs. If your code calls `signal(n, SIG_DFL)`
  /// to reset the signal handler for some signal _n_, the system will remove
  /// the signal handler installed by the testing library.
  ///
  /// If you cannot avoid resetting the signal handler in an exit test to
  /// `SIG_DFL`, pass ``failure`` instead of ``signal(_:)`` when you call
  /// ``expect(exitsWith:_:sourceLocation:performing:)`` or
  /// ``require(exitsWith:_:sourceLocation:performing:)``.

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[hjyamauchi]

@grynspan I'm not sure but does it looks like the Windows CI broke: https://ci-external.swift.org/job/swift-main-windows-toolchain/734/consoleText?

C:\Users\swift-ci\jenkins\workspace\swift-main-windows-toolchain\swift-testing\Sources\Testing\Support\Additions\WinSDKAdditions.swift:27:13: error: cannot convert value of type 'Int32' to expected argument type 'Int'
25 | 
26 |   // Set the severity and status bits.
27 |   result |= STATUS_SEVERITY_ERROR << 30
   |             `- error: cannot convert value of type 'Int32' to expected argument type 'Int'
28 |   result |= 1 << 29 // "Customer" bit
29 | 

C:\Users\swift-ci\jenkins\workspace\swift-main-windows-toolchain\swift-testing\Sources\Testing\Support\Additions\WinSDKAdditions.swift:27:35: error: cannot convert value of type 'Int32' to expected argument type 'Int'
25 | 
26 |   // Set the severity and status bits.
27 |   result |= STATUS_SEVERITY_ERROR << 30
   |                                   `- error: cannot convert value of type 'Int32' to expected argument type 'Int'
28 |   result |= 1 << 29 // "Customer" bit
29 | 

C:\Users\swift-ci\jenkins\workspace\swift-main-windows-toolchain\swift-testing\Sources\Testing\ExitTests\ExitTest.swift:224:74: error: cannot convert value of type 'NTSTATUS' (aka 'Int') to expected argument type 'Int32'
222 |     // For an explanation of this magic, see the corresponding logic in
223 |     // ExitTest.callAsFunction().
224 |     if case let .exitCode(exitCode) = result.exitCondition, (exitCode & ~STATUS_CODE_MASK) == STATUS_SIGNAL_CAUGHT_BITS {
    |                                                                          `- error: cannot convert value of type 'NTSTATUS' (aka 'Int') to expected argument type 'Int32'
225 |       result.exitCondition = .signal(exitCode & STATUS_CODE_MASK)
226 |     }

C:\Users\swift-ci\jenkins\workspace\swift-main-windows-toolchain\swift-testing\Sources\Testing\ExitTests\ExitTest.swift:224:71: error: binary operator '&' cannot be applied to two 'CInt' (aka 'Int32') operands
222 |     // For an explanation of this magic, see the corresponding logic in
223 |     // ExitTest.callAsFunction().
224 |     if case let .exitCode(exitCode) = result.exitCondition, (exitCode & ~STATUS_CODE_MASK) == STATUS_SIGNAL_CAUGHT_BITS {
    |                                                                       |- error: binary operator '&' cannot be applied to two 'CInt' (aka 'Int32') operands
    |                                                                       `- note: overloads for '&' exist with these partially matching parameter lists: (Int, Int), (Int32, Int32)
225 |       result.exitCondition = .signal(exitCode & STATUS_CODE_MASK)
226 |     }

C:\Users\swift-ci\jenkins\workspace\swift-main-windows-toolchain\swift-testing\Sources\Testing\ExitTests\ExitTest.swift:225:49: error: cannot convert value of type 'NTSTATUS' (aka 'Int') to expected argument type 'Int32'
223 |     // ExitTest.callAsFunction().
224 |     if case let .exitCode(exitCode) = result.exitCondition, (exitCode & ~STATUS_CODE_MASK) == STATUS_SIGNAL_CAUGHT_BITS {

225 |       result.exitCondition = .signal(exitCode & STATUS_CODE_MASK)

    |                                                 `- error: cannot convert value of type 'NTSTATUS' (aka 'Int') to expected argument type 'Int32'

226 |     }

227 | #endif

[grynspan]

That has already been resolved. See #776.

[hjyamauchi]

Thanks @grynspan !!