Use a StringRef. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220327 91177308-0d34-0410-b5e6-96231b3b80d8
R600: Use default GlobalDirective

The overridden one wasn't inserting a space,
so you would end up with .globalfoo

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220329 91177308-0d34-0410-b5e6-96231b3b80d8