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

@@ -152,12 +152,32 @@ extension BuildParameters {
             args = ["-alias", "_\(target.c99name)_main", "_main"]
         case .elf:
             args = ["--defsym", "main=\(target.c99name)_main"]
+        case .coff:
+            // If the user is specifying a custom entry point name that isn't "main", assume they may be setting WinMain or wWinMain
+            // and don't do any modifications ourselves. In that case the linker will infer the WINDOWS subsystem and call WinMainCRTStartup,
+            // which will then call the custom entry point. And WinMain/wWinMain != main, so this still won't run into duplicate symbol
+            // issues when called from a test target, which always uses main.
+            if let customEntryPointFunctionName = findCustomEntryPointFunctionName(of: target), customEntryPointFunctionName != "main" {
+                return nil
+            }
+            args = ["/ALTERNATENAME:main=\(target.c99name)_main", "/SUBSYSTEM:CONSOLE"]
         default:
             return nil
         }
         return args.asSwiftcLinkerFlags()
     }
 
+    private func findCustomEntryPointFunctionName(of target: ResolvedModule) -> String? {
+        let flags = createScope(for: target).evaluate(.OTHER_SWIFT_FLAGS)
+        var it = flags.makeIterator()
+        while let value = it.next() {
+            if value == "-Xfrontend" && it.next() == "-entry-point-function-name" && it.next() == "-Xfrontend" {
+                return it.next()
+            }
+        }
+        return nil
+    }
+
     /// Returns the scoped view of build settings for a given target.
     func createScope(for target: ResolvedModule) -> BuildSettings.Scope {
         BuildSettings.Scope(target.underlying.buildSettings, environment: buildEnvironment)

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()