[C++20] [Modules] [ClangScanDeps] Add ClangScanDeps support for C++20 Named...

[C++20] [Modules] [ClangScanDeps] Add ClangScanDeps support for C++20 Named Modules in P1689 format (2/4)

Close https://github.com/llvm/llvm-project/issues/51792
Close https://github.com/llvm/llvm-project/issues/56770

This patch adds ClangScanDeps support for C++20 Named Modules in P1689
format. We can find the P1689 format at:
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1689r5.html.
After we land the patch, we're able to compile C++20 Named
Modules with CMake! And although P1689 is written by kitware people,
other build systems should be able to use the format to compile C++20
Named Modules too.

TODO: Support header units in P1689 Format.
TODO2: Support C++20 Modules in the full dependency format of
ClangScanDeps. We also want to support C++20 Modules and clang modules
together according to
https://discourse.llvm.org/t/how-should-we-support-dependency-scanner-for-c-20-modules/66027.
But P1689 format cares about C++20 Modules only for now. So let's focus
on C++ Modules and P1689 format. And look at the full dependency format
later.

I'll add the ReleaseNotes and Documentations after the patch get landed.

Reviewed By: jansvoboda11

Differential Revision: https://reviews.llvm.org/D137527