[DepScanner] Do not remove the resource directory path

The clang importer sets the clang resource directory to
`RuntimeResourcePath`/clang (usually "lib/swift/clang", a symbolic link
into "lib/clang/<version>"). This was inadvertently being removed in a
check to remove the clang *executable* path.

Modify that check to only the first argument so the resource directory
is properly passed through.

Author: bnbarham

lib/ClangImporter/ClangModuleDependencyScanner.cpp

@@ -213,22 +213,29 @@ void ClangImporter::recordModuleDependencies(
       swiftArgs.push_back("-Xcc");
       swiftArgs.push_back(arg.str());
     };
-    // Add all args inheritted from creating the importer.
+
+    // Add all args inherited from creating the importer.
     auto It = allArgs.begin();
+
+    {
+      StringRef arg = *It;
+      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;
+      }
+    }
+
     while(It != allArgs.end()) {
       StringRef arg = *It;
       // Remove the -target arguments because we should use the target triple
       // specified with `-clang-target` on the scanner invocation, or
       // 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

@@ -126,7 +126,11 @@ import SubE
 // CHECK: "commandLine": [
 // CHECK-NEXT: "-frontend"
 // CHECK-NEXT: "-only-use-extra-clang-opts"
-// CHECK-NOT: "BUILD_DIR/bin/clang"
+// CHECK-NOT:  "BUILD_DIR/bin/clang"
+// CHECK:      "-Xcc"
+// CHECK-NEXT: "-resource-dir"
+// CHECK-NEXT: "-Xcc"
+// CHECK-NEXT: "BUILD_DIR/lib/swift/clang"
 // CHECK:      "-fsystem-module",
 // CHECK-NEXT: "-emit-pcm",
 // CHECK-NEXT: "-module-name",