[clang] [Clang] Ignore -fchar8_t in C (PR #138716)

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cor3ntin wrote:

This is part of the problem, yes.
we also do not support char16_t/char32_t as keywords in C and u8 behaves differently.
And of course, there is no way to use it with the standard library or... any library?


Moreover we have 0 tests for C so I don't think we can claim to support that as an extension even if we wanted to. It would take significantly more effort than having a flag.

Afaict the support was purely accidental

https://github.com/llvm/llvm-project/pull/138716