[LinkerWrapper][NFC] Change interface to use a StringRef to TempFiles

Summary:
Currently we use temporary files to write the intermediate results to.
However, these are stored as regular strings and we do a few unnecessary
copies and conversions of them. This patch simply replaces these strings
with a reference to the filename stored in the list of temporary files.
The temporary files will stay alive during the whole linking phase and
have stable pointers, so we should be able to cheaply pass references to
them rather than copying them every time.