Revert "Query the StringMap only once when creating MDString (NFC)"

This reverts commit r261030 and r261036.
(The revision was marked "approved" on phabricator, but some concerns
were raised on the mailing list. Thanks D. Blaikie for notifying me.)

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 261055