fix a memory leak yjasskin pointed out: MCSymbol is bump pointer

fix a memory leak yjasskin pointed out: MCSymbol is bump pointer
allocated and thus not freed.  This is cool except that it contains
and std::string so the string data didn't get freed.  In any case
there is no reason to redundantly store the string data in the
MCSymbol anyway, just make the MCSymbol ref the string data in the
MCContext StringMap.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@98536 91177308-0d34-0410-b5e6-96231b3b80d8