Add GUID/getGlobalIdentifier() non-static API to global value

Summary:
These are just helpers calling their static counter part to
simplify client code.

Reviewers: tejohnson

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D18339

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 264382