Remember that we have a null terminated string.

This is a micro optimization. Instead of going char*->StringRef->Twine->char*,
go char*->Twine->char* and avoid having to copy the filename on the stack.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186380 91177308-0d34-0410-b5e6-96231b3b80d8