[unix] Fix linker argument in Unix toolchains

The spelling of the linker is `-fuse-ld=`, not `--fuse-ld=`, as it can
be seen in [1] and as used in the C++ driver in [2].

The code was correct before swiftlang#1545 using `-fuse-ld=`, but was changed to
`--fuse-ld=`. The test was testing for the command line containing the
same spelling, so it would had never failed, but running the actual
command line would had failed. That code was eventually reverted and
reverted back in swiftlang#1608.

[1]: https://github.com/apple/llvm-project/blob/e33819dff1cbf7a90b0216a4993126cb11440d45/clang/include/clang/Driver/Options.td#L5267
[2]: https://github.com/apple/swift/blob/319f36b01baa082fd2d2640d6e320594752702d0/lib/Driver/UnixToolChains.cpp#L234

Author: drodriguez

Sources/SwiftDriver/Jobs/GenericUnixToolchain+LinkerSupport.swift

@@ -50,9 +50,9 @@ extension GenericUnixToolchain {
     case .executable:
       // Select the linker to use.
       if let arg = parsedOptions.getLastArgument(.useLd)?.asSingle {
-        commandLine.appendFlag("--fuse-ld=\(arg)")
+        commandLine.appendFlag("-fuse-ld=\(arg)")
       } else if lto != nil {
-        commandLine.appendFlag("--fuse-ld=lld")
+        commandLine.appendFlag("-fuse-ld=lld")
       }
 
       if let arg = parsedOptions.getLastArgument(.ldPath)?.asSingle {

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -2372,7 +2372,7 @@ final class SwiftDriverTests: XCTestCase {
       let plannedJobs = try driver.planBuild().removingAutolinkExtractJobs()
       let lastJob = plannedJobs.last!
       XCTAssertTrue(lastJob.tool.name.contains("clang"))
-      XCTAssertTrue(lastJob.commandLine.contains(.flag("--fuse-ld=lld")))
+      XCTAssertTrue(lastJob.commandLine.contains(.flag("-fuse-ld=lld")))
     }
   }