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

[Hassan Sajjad via cfe-commits]

HassanSajjad-302 wrote:

Hi. I opened this [RFC](https://discourse.llvm.org/t/need-help-to-add-scanning-less-c-20-header-units-and-modules-build-support/87323). I think I titled it incorrectly because of which there has been less activity. But I could not edit the title.

I don't know how I can make this patch smaller. Its size mostly comes from the library size of [ipc2978api](https://github.com/HassanSajjad-302/ipc2978api) and its integration in the clang code. 

In past month, I was working on my build-system and hope to complete this feature in next week. After which I would complete this patch to further testing and optimization with Boost as example.

I would appreciate if help could be provided in completing this and a Clang maintainer reviews this. This does not need to be merged **yet**. However a review from Clang maintainer would add a lot of weight which could help in HMake adoption by Boost, after which a case can be made of this merging here.

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