[clang] NFCI: Make `ModuleFile::File` non-optional (#74892)

AFAICT, `ModuleFile::File` can be `std::nullopt` only for PCM files
loaded from the standard input. This patch starts setting that variable
to `FileManager::getSTDIN()` in that case, which makes it possible to
remove the optionality, and also simplifies code that actually reads the
file.

This is part of an effort to get rid of
`Optional{File,Directory}EntryRefDegradesTo{File,Directory}EntryPtr`.