[pr20127] Check for leading \1 in the Twine version of getNameWithPrefix.

No functionality change, but will simplify an upcoming patch that uses the
Twine version.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214515 91177308-0d34-0410-b5e6-96231b3b80d8