Add support for testable executables on Windows

This uses the /ALTERNATENAME flag to link.exe, which is roughly equivalent to -alias/--defsym. This has been verified to work in a sample project.

See https://devblogs.microsoft.com/oldnewthing/20200731-00/ for more info.

Closes #6367

Author: jakepetroules

Sources/Build/BuildPlan/BuildPlan.swift

@@ -151,6 +151,8 @@ extension BuildParameters {
             args = ["-alias", "_\(target.c99name)_main", "_main"]
         case .elf:
             args = ["--defsym", "main=\(target.c99name)_main"]
+        case .coff:
+            args = ["/ALTERNATENAME:main=\(target.c99name)_main", "/SUBSYSTEM:CONSOLE"]
         default:
             return nil
         }

Tests/BuildTests/BuildPlanTests.swift

@@ -3875,7 +3875,7 @@ class BuildPlanTestCase: BuildSystemProviderTestCase {
                 observabilityScope: observability.topScope
             ))
         }
-        let supportingTriples: [Basics.Triple] = [.x86_64Linux, .x86_64MacOS]
+        let supportingTriples: [Basics.Triple] = [.x86_64Linux, .x86_64MacOS, .x86_64Windows]
         for triple in supportingTriples {
             let result = try await createResult(for: triple)
             let exe = try result.moduleBuildDescription(for: "exe").swift().compileArguments()
@@ -3884,7 +3884,7 @@ class BuildPlanTestCase: BuildSystemProviderTestCase {
             XCTAssertMatch(linkExe, [.contains("exe_main")])
         }
 
-        let unsupportingTriples: [Basics.Triple] = [.wasi, .windows]
+        let unsupportingTriples: [Basics.Triple] = [.wasi]
         for triple in unsupportingTriples {
             let result = try await createResult(for: triple)
             let exe = try result.moduleBuildDescription(for: "exe").swift().compileArguments()