[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/DriverTool/swift_cache_tool_main.cpp

@@ -194,7 +194,7 @@ private:
                              MainExecutablePath))
       return true;
 
-    if (!Invocation.getFrontendOptions().EnableCAS) {
+    if (!Invocation.getFrontendOptions().EnableCaching) {
       llvm::errs() << "Requested command-line arguments do not enable CAS\n";
       return true;
     }