Implement -debug-module-path for implicit modules in swift-driver

There is no fundamental technical reason to restrict this feature to
explict module only, so this patch adds support for passing the
correct -debug-module-path also to implicit builds.

rdar://168256846

Author: adrian-prantl

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -109,12 +109,23 @@ extension Driver {
       jobNeedPathRemap = false
     }
 
+    // Add the -debug-module-path option to the compile job.
     if isPlanJobForExplicitModule && forObject && isFrontendArgSupported(.debugModulePath),
        let explicitModulePlanner {
       let mainModule = explicitModulePlanner.dependencyGraph.mainModule
       let pathHandle = moduleOutputInfo.output?.outputPath ?? mainModule.modulePath.path
       let path = VirtualPath.lookup(pathHandle)
       try addPathOption(option: .debugModulePath, path: path, to: &commandLine, remap: jobNeedPathRemap)
+    } else if !isPlanJobForExplicitModule && isFrontendArgSupported(.debugModulePath),
+      let pathHandle = moduleOutputInfo.output?.outputPath {
+      // Recompute the module path based on the module name, because
+      // this is effectively passing the ouput of the module merge
+      // action which depends on the compile action.
+      let path = try VirtualPath.lookup(pathHandle)
+        .parentDirectory.appending(component: moduleOutputInfo.name)
+        .replacingExtension(with: .swiftModule)
+        .intern()
+      try addPathOption(option: .debugModulePath, path: VirtualPath.lookup(path), to: &commandLine, remap: jobNeedPathRemap)
     }
 
     // Check if dependency scanner has put the job into direct clang cc1 mode.

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -4118,6 +4118,21 @@ final class SwiftDriverTests: XCTestCase {
     }
   }
 
+
+  func testDebugModulePath() throws {
+    do {
+      var driver = try Driver(args: ["swiftc", "-target", "arm64-apple-macos26", "-g", "foo.swift"])
+      let plannedJobs = try driver.planBuild()
+      let mergeJob = try plannedJobs.findJob(.emitModule)
+      XCTAssertGreaterThan(mergeJob.outputs.count, 0)
+      let moduleFile = mergeJob.outputs[0]
+      XCTAssertEqual(moduleFile.type, .swiftModule)
+      let compileJob = try plannedJobs.findJob(.compile)
+      // This needs to be "foo.swiftmodule", not the unmerged "foo-1.swiftmodule".
+      XCTAssertJobInvocationMatches(compileJob, .flag("-debug-module-path"), .path(try .temporary(RelativePath(validating: "foo")).replacingExtension(with: .swiftModule)))
+    }
+  }
+
   func testModuleWrapJob() throws {
     // FIXME: These tests will fail when run on macOS, because
     // swift-autolink-extract is not present