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.

llvm-svn: 219224