[clang] [C++20] [Modules] Embed all source files for C++20 Modules (PR #102444)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

FWIW, I meant to review this, but somehow this disappeared from my waiting for review list and I lost track of it :)

https://github.com/llvm/llvm-project/pull/102444