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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > Was this discussed/reviewed/motivated? There are drawbacks to this approach outlined in #72383
> > @iains @jyknight @AaronBallman @Bigcheese
> 
> The motivation is in #72383 and I comment in [#72383 (comment)](https://github.com/llvm/llvm-project/issues/72383#issuecomment-2275135890)
> 
> This is not reviewed. I wait for several weeks but got no response. And I think it is good. So I choose to land it.

Thank you for the link! (FWIW, there's no problems with these changes having been landed; @ChuanqiXu9 is the code owner for modules and the PR was up for three weeks without discussion. This is just typical post-commit review feedback.)

There's a fair amount of discussion on that thread in opposition to this approach, and a comment thread on an issue is not really visible to many people. I think this warrants an RFC for a broader discussion, so I'd appreciate temporarily reverting this patch.

Some thoughts for the RFC discussion:

* Given that this is not the default behavior for MSVC, should `clang-cl` behave differently than `clang`?
* What are the impacts on other tooling (debugger, 3rd party static analysis, etc)?
* Is this the correct default when considering intellectual property security (will people expect their full, original source to be something that can be pulled from these artifacts)?

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