[Dependency Scanning] Emit path to clang from PCM build command-line

It gets interpreted as just an executable we feed to the linker, since we launch `emit-pcm` actions via `swift-frontend` invocations, not `clang`.

Author: artemcm

lib/ClangImporter/ClangModuleDependencyScanner.cpp

@@ -242,6 +242,13 @@ void ClangImporter::recordModuleDependencies(
       // from the depending Swift modules.
       if (arg == "-target") {
         It += 2;
+      } else if (arg == "clang" ||
+                 arg.endswith(llvm::sys::path::get_separator().str() + "clang")) {
+        // Remove the initial path to clang executable argument, to avoid
+        // treating it as an executable input to compilation. It is not needed
+        // because the consumer of this command-line will invoke the emit-PCM
+        // action via swift-frontend.
+        It += 1;
       } else if (arg.startswith("-fapinotes-swift-version=")) {
         // Remove the apinotes version because we should use the language version
         // specified in the interface file.

test/ScanDependencies/module_deps.swift

@@ -126,8 +126,7 @@ import SubE
 // CHECK: "commandLine": [
 // CHECK-NEXT: "-frontend"
 // CHECK-NEXT: "-only-use-extra-clang-opts"
-// CHECK-NEXT: "-Xcc"
-// CHECK-NEXT: "BUILD_DIR/bin/clang"
+// CHECK-NOT: "BUILD_DIR/bin/clang"
 // CHECK:      "-fsystem-module",
 // CHECK-NEXT: "-emit-pcm",
 // CHECK-NEXT: "-module-name",

test/ScanDependencies/module_deps_cache_reuse.swift

@@ -109,9 +109,8 @@ import SubE
 
 // CHECK: "commandLine": [
 // CHECK-NEXT: "-frontend"
-// CHECK-NEXT: "-only-use-extra-clang-opts"
-// CHECK-NEXT: "-Xcc"
-// CHECK-NEXT: "BUILD_DIR/bin/clang"
+// CHECK-NEXT: "-only-use-extra-clang-opts
+// CHECK-NOT: "BUILD_DIR/bin/clang"
 // CHECK-NEXT: "-Xcc"
 // CHECK-NEXT: "-fsyntax-only",
 // CHECK:      "-fsystem-module",