Frontend: Migrate to FileEntryRef in TextDiagnosticTest, NFC

Migrate over to the `FileEntryRef` overloads of
`SourceManager::createFileID` and `overrideFileContents` (using
`getVirtualFileRef`) in `TextDiagnostic`'s `ShowLine` test.

No functionality change.

Differential Revision: https://reviews.llvm.org/D92968