SourceManager: Unify FileEntry/FileEntryRef versions of createFileID

Change `SourceManager::createFileID(const FileEntry*)` to defer to
`SourceManager::createFileID(FileEntryRef)`. This fixes an unexercised
bug where the latter gained support for named pipes and the former
didn't, but since we're trying to remove all calls to the former it
doesn't really make sense to test this explicitly now that the
implementation is hollowed out.

This is a belated follow-up to 245218bb,
which sunk named pipe support into FileManager and SourceManager. The
original version of that patch was based on top of
https://reviews.llvm.org/D92984, which removed the `FileEntry` overload
of `createFileID()`, and I missed the subtle difference when it was
rebased.