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

[Aaron Ballman via cfe-commits]

================
@@ -0,0 +1,29 @@
+// RUN: %clang_cc1 -verify=c23 -std=c23 %s
+// RUN: %clang_cc1 -verify=c17 -std=c17 %s
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AaronBallman wrote:

I don't think we should remove the test for stdatomic,h, that's an important property for us to test and may be papering over a bug here. The only situation we should be getting the system header is:
```
#if __STDC_HOSTED__ &&                                                         \
    __has_include_next(<stdatomic.h>) &&                                       \
    (!defined(_MSC_VER) || (defined(__cplusplus) && __cplusplus >= 202002L))
# include_next <stdatomic.h>
#else
...
so `-ffreestanding` really should only include our header. What do you get from `-E` on that machine? Are we actually including the system header anyway, or did we mess up our header somehow?
```

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