[clang] Add -fsanitize=memtag-globals (no-op).

Adds the -fsanitize plumbing for memtag-globals. Makes -fsanitize=memtag
imply -fsanitize=memtag-globals.

This has no effect on codegen for now.

Reviewed By: eugenis, aaron.ballman

Differential Revision: https://reviews.llvm.org/D127163