Add missing const to StringRef.copy()

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216811 91177308-0d34-0410-b5e6-96231b3b80d8
Fixed a build problem when there were headers
for a different LLVM present in the system header
lookup path.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216812 91177308-0d34-0410-b5e6-96231b3b80d8