[CodeGen] Add an option to suppress output of llvm.ident

Summary:
By default Clang outputs its version (including git commit hash, in
case of trunk builds) into object and assembly files. It might be
useful to have an option to disable this, especially for debugging
purposes.
This patch implements new command line flags -Qn and -Qy (the names
are chosen for compatibility with GCC). -Qn disables output of
the 'llvm.ident' metadata string and the 'producer' debug info. -Qy
(enabled by default) does the opposite.

Reviewers: faisalv, echristo, aprantl

Reviewed By: aprantl

Subscribers: aprantl, cfe-commits, JDevlieghere, rogfer01

Differential Revision: https://reviews.llvm.org/D45255

llvm-svn: 330442