[Bash-autocompletion] Auto complete cc1 options if -cc1 is specified

Summary:
We don't want to autocomplete flags whose Flags class has `NoDriverOption` when argv[1] is not `-cc1`.

Another idea for this implementation is to make --autocomplete a cc1
option and handle it in clang Frontend, by porting --autocomplete
handler from Driver to Frontend, so that we can handle Driver options
and CC1 options in unified manner.

Differential Revision: https://reviews.llvm.org/D34770

llvm-svn: 307479