StringMap<DIE*>::iterator::first() returns a pointer to the first character of

StringMap<DIE*>::iterator::first() returns a pointer to the first character of
the key. This will cause it to create a new std::string, which isn't
wanted. Instead, pass back the "const char*". Modify the EmitString() method to
take a "const char*".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@68741 91177308-0d34-0410-b5e6-96231b3b80d8