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

There is no fundamental technical reason to restrict this feature to
explict modules 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 +109,25 @@ extension Driver {
       jobNeedPathRemap = false
     }
 
-    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)
+    // Add the -debug-module-path option to the compile job.
+    if forObject && isFrontendArgSupported(.debugModulePath) {
+      let modulePathHandle : VirtualPath.Handle
+      if isPlanJobForExplicitModule,
+         let explicitModulePlanner {
+        let mainModule = explicitModulePlanner.dependencyGraph.mainModule
+        modulePathHandle = moduleOutputInfo.output?.outputPath ?? mainModule.modulePath.path
+      } else {
+        // Recompute the module path based on the module name, because this is effectively passing the output
+        // of the module merge action which depends on the compile action.
+        let moduleBase : VirtualPath
+        if let pathHandle = moduleOutputInfo.output?.outputPath {
+          moduleBase = VirtualPath.lookup(pathHandle).parentDirectory.appending(component: moduleOutputInfo.name)
+        } else {
+          moduleBase = .relative(try RelativePath(validating: moduleOutputInfo.name))
+        }
+        modulePathHandle = try moduleBase.replacingExtension(with: .swiftModule).intern()
+      }
+      try addPathOption(option: .debugModulePath, path: VirtualPath.lookup(modulePathHandle), to: &commandLine, remap: jobNeedPathRemap)
     }
 
     // Check if dependency scanner has put the job into direct clang cc1 mode.

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -4118,6 +4118,26 @@ final class SwiftDriverTests: XCTestCase {
     }
   }
 
+
+  func testDebugModulePath() throws {
+    do {
+      var driver = try Driver(args: ["swiftc", "-target", "arm64-unknown-macos26", "-g", "foo.swift"])
+      let plannedJobs = try driver.planBuild()
+      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)))
+    }
+    // Unfortunately this depends on dsymutil.
+    #if os(macOS)
+    do {
+      var driver = try Driver(args: ["swiftc", "-target", "arm64-unknown-macos26", "-g", "foo.swift"])
+      let plannedJobs = try driver.planBuild()
+      let compileJob = try plannedJobs.findJob(.compile)
+      XCTAssertJobInvocationMatches(compileJob, .flag("-debug-module-path"), .path(try .temporary(RelativePath(validating: "foo")).replacingExtension(with: .swiftModule)))
+    }
+  #endif
+  }
+
   func testModuleWrapJob() throws {
     // FIXME: These tests will fail when run on macOS, because
     // swift-autolink-extract is not present