Hash Metadata using pointer for MDString argument instead of value (NFC)

MDString are uniqued in the Context on creation, hashing the
pointer is less expensive than hashing the String itself.

Reviewers: dexonsmith
Differential Revision: http://reviews.llvm.org/D16560

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 263867