[clang][nfc] refactor Module::Header to use OptionalFileEntryRef

Refactor the `Module::Header` class to use an `OptionalFileEntryRef`
instead of a `FileEntry*`. This is preparation for refactoring the
`TopHeaderNames` to use `FileEntryRef` so that we preserve the
lookup path of the headers when serializing.

This is mostly based on https://reviews.llvm.org/D90497

Reviewed By: jansvoboda11

Differential Revision: https://reviews.llvm.org/D142113