[clang] [libcxx] [llvm] [Clang] Add warnings when mixing different charN_t types (PR #138708)

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https://github.com/EricWF commented:

I don't mean to be difficult, but I'm always weary about removing test coverage, and I'm not yet convinced that the tests didn't intend to test these sketchy conversions.

The way it uses the typedefs elsewhere in the same tests, while pinning some of the types initially to `char16_t`, for example, suggests to me it was intentional (though not how you would want users to do it).

I'm happy to proceed without addressing my concerns if another expert can vouch for the test changes, otherwise I'm sure I'll be convinced by the answers to the inline comments.

Apologies for my paranoia.

https://github.com/llvm/llvm-project/pull/138708