[clang][modules] NFCI: Extract optionality out of `Module::{Header,DirectoryName}`

Most users of `Module::Header` already assume its `Entry` is populated. Enforce this assumption in the type system and handle the only case where this is not the case by wrapping the whole struct in `std::optional`. Do the same for `Module::DirectoryName`.

Depends on D151584.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D151586