Be consistent about using "const Twine &" for filenames.

On this file we had a mix of
* Twine
* const char *
* StringRef

The two that make sense are
* const Twine & (caller convenience)
* consc char * (that is what will eventually be passed to open.

Given that sys::fs::openFileForRead takes a "const Twine &", I picked that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219224 91177308-0d34-0410-b5e6-96231b3b80d8