ExpressionParser: Migrate to FileEntryRef in ParseInternal, NFC

Migrate to the `FileEntryRef` overload of `SourceManager::createFileID`
(using `FileManager::getOptionalFileRef`) in
`ClangExpressionParser::ParseInternal`.

No functionality change here.

Differential Revision: https://reviews.llvm.org/D92957

GitOrigin-RevId: 8b6aedc4c99a8419bc27949c7b9eb18af4cd486b
1 file changed