[6.0 🍒][Dependency Scanning] Specify Source Locations For Missing Module Dependencies

[artemcm]

Cherry-pick of #73600

Explanation: This change modifies the dependency scanner to keep track of source locations of each encountered import statement, in order to be able to emit diagnostics with source locations if an import failed to resolve.

• Keep track of each 'import' statement's source buffer, line number, and column number when adding it. The dependency scanner utilizes separate compilation instances, and therefore separate Source Managers for scanning import statements of user sources and textual interfaces of Swift dependencies. Since import resolution may happen in the main scanner compilation instance while the import itself was found by an interface-scanning sub-instance, we cannot simply hold on to the import's SourceLoc.
• Add libSwiftScan API for diagnostics to carry above source locations to clients.

Now instead of un-locatable:

<unknown>0 error: Unable to find module dependency: 'Baz'

we will now get rich diagnostic output:

/.../test.swift:2:8: error: Unable to find module dependency: 'Baz'
 1 | import A
 2 | import Baz
   |        |- error: Unable to find module dependency: 'Baz'
   |        `- note: a dependency of main module 'test'
 3 | import C
 4 | import Baz
   |        `- note: also imported here

Risk: Low-Moderate. This only affects projects which use dependency scanning with Explicitly-Built Modules. The added locations are only used when emitting errors on already-failing compilations.

Testing: Automated tests added to the compiler and driver suite.
Reviewed By: @owenv @cachemeifyoucan @nkcsgexi

[artemcm]

swiftlang/swift-driver#1620
@swift-ci test

[nkcsgexi]

Thank you🎉!