[clang] [Clang] [C23] Implement N2653: u8 strings are char8_t[] (PR #97208)

[Mital Ashok via cfe-commits]

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@@ -104,6 +107,7 @@ typedef _Atomic(long)               atomic_long;
 typedef _Atomic(unsigned long)      atomic_ulong;
 typedef _Atomic(long long)          atomic_llong;
 typedef _Atomic(unsigned long long) atomic_ullong;
+typedef _Atomic(unsigned char)      atomic_char8_t;
 typedef _Atomic(uint_least16_t)     atomic_char16_t;
 typedef _Atomic(uint_least32_t)     atomic_char32_t;
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MitalAshok wrote:

Should the definition of `atomic_char8_t` also be gated behind C23/C++20?

Also in C++20, this should be `_Atomic(char8_t)`, but the next two are also wrong in C++11 (`_Atomic(char16_t)`/`_Atomic(char32_t)`)

https://github.com/llvm/llvm-project/pull/97208