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

[David Blaikie via cfe-commits]

dwblaikie wrote:

FWIW, we saw failures at Google (where, to the best of my knowledge, we aren't using named modules at all) that look like this:
```
error: '#include <filename>' attaches the declarations to the named module '.get', which is not usually intended; consider moving that directive before the module declaration [-Werror,-Winclude-angled-in-module-purview]
   14 | #include <stddef.h>
```
So there's probably some problems with this patch? is this enough to go on, or would a reduced test case be required to address the issue? (reducing modules issues is a bit difficult/expensive, or I'd have provided it up front)


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