Add driver options to swift to enable MCCAS.

To enable MCCAS, the following driver options have been added

-cas-backend: Enable MCCAS backend in swift, the option
-cache-compile-job must also be used.

-cas-backend-mode=native: Set the CAS Backend mode to emit an object
file after materializing it from the CAS.

-cas-backend-mode=casid: Emit a file with the CASID for the CAS that was
created.

-cas-backend-mode=verify: Verify that the object file created is
identical to the object file materialized from the CAS.

-cas-emit-casid-file: Emit a .casid file next to the object file when
CAS Backend is enabled.

lib/Frontend/CompilerInvocation.cpp

@@ -266,6 +266,10 @@ setIRGenOutputOptsFromFrontendOptions(IRGenOptions &IRGenOpts,
     }
   }(FrontendOpts.RequestedAction);
 
+  IRGenOpts.UseCASBackend = FrontendOpts.UseCASBackend;
+  IRGenOpts.CASObjMode = FrontendOpts.CASObjMode;
+  IRGenOpts.EmitCASIDFile = FrontendOpts.EmitCASIDFile;
+
   // If we're in JIT mode, set the requisite flags.
   if (FrontendOpts.RequestedAction == FrontendOptions::ActionType::Immediate) {
     IRGenOpts.UseJIT = true;
@@ -1440,7 +1444,8 @@ static bool ParseClangImporterArgs(ClangImporterOptions &Opts,
                                    DiagnosticEngine &Diags,
                                    StringRef workingDirectory,
                                    const LangOptions &LangOpts,
-                                   const FrontendOptions &FrontendOpts) {
+                                   const FrontendOptions &FrontendOpts,
+                                   bool RequiresCAS) {
   using namespace options;
 
   if (const Arg *a = Args.getLastArg(OPT_tools_directory)) {
@@ -1568,7 +1573,7 @@ static bool ParseClangImporterArgs(ClangImporterOptions &Opts,
 
   // Forward the FrontendOptions to clang importer option so it can be
   // accessed when creating clang module compilation invocation.
-  if (FrontendOpts.EnableCaching)
+  if (RequiresCAS)
     Opts.CASOpts = FrontendOpts.CASOpts;
 
   return false;
@@ -3025,7 +3030,7 @@ bool CompilerInvocation::parseArgs(
   }
 
   if (ParseClangImporterArgs(ClangImporterOpts, ParsedArgs, Diags,
-                             workingDirectory, LangOpts, FrontendOpts)) {
+                             workingDirectory, LangOpts, FrontendOpts, requiresCAS())) {
     return true;
   }