[clang] Added partial support for compiling C++20 modules and header-units without scanning. (PR #147682)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Simplified quite a lot and based it on the latest commit. Lots of new features have been added in HMake including the zero-cost Adaptive Jumbo Build. Please give this a read ( 2min) [HassanSajjad-302/HMake#4](https://github.com/HassanSajjad-302/HMake/discussions/4)

I am re-closing this PR because the RFC for this functionality [did not have consensus](https://discourse.llvm.org/t/rfc-hmake-for-llvm/88997/16). Please respect the community's limited bandwidth for review efforts and do not reopen the PR to ask the community to review changes on it.

https://github.com/llvm/llvm-project/pull/147682