[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. Now, I have complete 90% of N2978 with these changes. I also extended the test. Only shared file PCM feature is left. I will now work on HMake and will test the boost with this commit.

I added new optimizations in HMake. With these, HMake is now 2x faster and consumes 2x-3x less memory than Ninja in zero-target build. These will be reflected with no scan build as well.

Please review.

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