[ScanDependency] Add -experimental-clang-importer-direct-cc1-scan

Add an experimental option to tell dependency scanner to report clang
cc1 args should be used to construct clang importer in all constructed
swift-frontend tasks.

include/swift/Basic/LangOptions.h

@@ -969,6 +969,10 @@ namespace swift {
     /// Using ClangIncludeTreeRoot for compilation.
     bool HasClangIncludeTreeRoot = false;
 
+    /// Whether the dependency scanner should construct all swift-frontend
+    /// invocations directly from clang cc1 args.
+    bool ClangImporterDirectCC1Scan = false;
+
     /// Return a hash code of any components from these options that should
     /// contribute to a Swift Bridging PCH hash.
     llvm::hash_code getPCHHashComponents() const {