[Driver] Alias -fvisibility=internal to -fvisibility=hidden

The ELF symbol visibilities are:
- internal: Not visibile across DSOs, cannot pass address across DSOs
- hidden: Not visibile across DSOs, can be called indirectly
- default: Usually visible across DSOs, possibly interposable
- protected: Visible across DSOs, not interposable

LLVM only supports the latter 3 visibilities. Internal visibility is in
theory useful, as it allows you to assume that the caller is maintaining
a PIC register for you in %ebx, or in some other pre-arranged location.
As far as LLVM is concerned, this isn't worth the trouble. Using hidden
visibility is always correct, so we can just do that.

Resolves PR9183.

llvm-svn: 250954