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

[Mital Ashok via cfe-commits]

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

That still seems to fail on the CI:

```
******************** TEST 'Clang :: C/C23/n2653.c' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 1: /var/lib/buildkite-agent/builds/linux-56-59b8f5d88-vhr99-1/llvm-project/github-pull-requests/build/bin/clang -cc1 -internal-isystem /var/lib/buildkite-agent/builds/linux-56-59b8f5d88-vhr99-1/llvm-project/github-pull-requests/build/lib/clang/19/include -nostdsysteminc -ffreestanding -verify=c23 -std=c23 /var/lib/buildkite-agent/builds/linux-56-59b8f5d88-vhr99-1/llvm-project/github-pull-requests/clang/test/C/C23/n2653.c
+ /var/lib/buildkite-agent/builds/linux-56-59b8f5d88-vhr99-1/llvm-project/github-pull-requests/build/bin/clang -cc1 -internal-isystem /var/lib/buildkite-agent/builds/linux-56-59b8f5d88-vhr99-1/llvm-project/github-pull-requests/build/lib/clang/19/include -nostdsysteminc -ffreestanding -verify=c23 -std=c23 /var/lib/buildkite-agent/builds/linux-56-59b8f5d88-vhr99-1/llvm-project/github-pull-requests/clang/test/C/C23/n2653.c
error: 'c23-error' diagnostics seen but not expected: 
  File /var/lib/buildkite-agent/builds/linux-56-59b8f5d88-vhr99-1/llvm-project/github-pull-requests/clang/test/C/C23/n2653.c Line 11: missing
1 error generated.

--

********************
```

So I've just removed the test for <stdatomic.h> (`-nostdinc` seems to disable the clang header too)

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