Turn some Twine locals into const char * variables.

No functionality change, just stylistic cleanup. Change made by clang-tidy
and clang-format.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212544 91177308-0d34-0410-b5e6-96231b3b80d8
Update unit test for signature change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212545 91177308-0d34-0410-b5e6-96231b3b80d8