[CAS] Allow uncached job from CAS based dependency scanning

Create a path that swift-frontend can execute an uncached job from
modules built with CAS based explicit module build. The new flag
-import-module-from-cas will allow an uncached build to load module
from CAS, and combined with source file from real file system to build
the current module. This allows quick iterations that bypasses CAS,
without full dependency scanning every time in between.

rdar://152441866

lib/Frontend/CompilerInvocation.cpp

@@ -800,6 +800,8 @@ static bool ParseCASArgs(CASOptions &Opts, ArgList &Args,
                                             std::string(Value));
   }
 
+  Opts.ImportModuleFromCAS |= Args.hasArg(OPT_module_import_from_cas);
+
   if (auto *A = Args.getLastArg(OPT_clang_include_tree_root))
     Opts.ClangIncludeTree = A->getValue();
   if (auto *A = Args.getLastArg(OPT_clang_include_tree_filelist))