[CAS] Consolidate compile cache flags

Rename `-enable-cas` to `-compile-cache-job` to align with clang option
names and promote that to a new driver only flag.

Few other additions to driver flag for caching behaviors:
* `-compile-cache-remarks`: now cache hit/miss remarks are guarded behind
  this flag
* `-compile-cache-skip`: skip replaying from the cache. Useful as a
  debugging tool to do the compilation using CAS inputs even the output
  is a hit from the cache.

lib/Frontend/CompilerInvocation.cpp

@@ -1505,7 +1505,7 @@ 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.EnableCAS)
+  if (FrontendOpts.EnableCaching)
     Opts.CASOpts = FrontendOpts.CASOpts;
 
   return false;