[clang] [C++20] [Modules] Warn if we found #include <filename> in module purview (PR #69555)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> (in this context) Framework headers are like system (or user headers) but searched by a different mechanism (so they are #include-ed). In principle, the same constraints should apply - <Framework/....> indicates a "system" header and probably should not be included in the module purview.

Got it. Then it looks like not a problem for Frameworks. Let's wait for the opinion from @Bigcheese 

https://github.com/llvm/llvm-project/pull/69555