[clang] [C2y] Claim conformance and add test coverage for WG14 N3346 (PR #115516)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-cmake-x86_64-avx512-linux` running on `avx512-intel64` while building `clang` at step 7 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/133/builds/6505

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 7 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'Clang :: C/C2y/n3346.c' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 1: /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/stage1/bin/clang -cc1 -internal-isystem /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/stage1/lib/clang/20/include -nostdsysteminc -verify -std=c2y -Wall -pedantic -ffreestanding /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/llvm/clang/test/C/C2y/n3346.c
+ /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/stage1/bin/clang -cc1 -internal-isystem /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/stage1/lib/clang/20/include -nostdsysteminc -verify -std=c2y -Wall -pedantic -ffreestanding /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/llvm/clang/test/C/C2y/n3346.c
error: 'expected-error' diagnostics expected but not seen: 
  File /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/llvm/clang/test/C/C2y/n3346.c Line 52: array initializer must be an initializer list
error: 'expected-error' diagnostics seen but not expected: 
  File /localdisk2/buildbot/llvm-worker/clang-cmake-x86_64-avx512-linux/llvm/clang/test/C/C2y/n3346.c Line 52: initializing wide char array with non-wide string literal
2 errors generated.

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/115516