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

[Iain Sandoe via cfe-commits]

https://github.com/iains commented:

When I suggested this, it was an intention to limit it to C++ std library headers.  Do we not have some existing mechanism that knows which headers are in this category (for typo correction etc?)

My concern with making it more general is that it then covers things like frameworks which are also included like `<Framework/....>` (of course, it's also probably not what was intended there as well)

Let's add @Bigcheese to the reviewers, for an opinion on Frameworks


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