Add sanitizer-specific GlobalValue attributes.

Plan is the migrate the global variable metadata for sanitizers, that's
currently carried around generally in the 'llvm.asan.globals' section,
onto the global variable itself.

This patch adds the attribute and plumbs it through the LLVM IR and
bitcode formats, but is a no-op other than that so far.

Reviewed By: vitalybuka, kstoimenov

Differential Revision: https://reviews.llvm.org/D126100