Query the StringMap only once when creating MDString (NFC)

Summary: Loading IR with debug info improves MDString::get() from 19ms to 10ms.

Reviewers: dexonsmith

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16597

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 261030