[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-debian-cpp20` running on `clang-debian-cpp20` while building `clang` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/108/builds/5730

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

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'Clang :: C/C2y/n3346.c' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 1: /vol/worker/clang-debian-cpp20/clang-debian-cpp20/build/bin/clang -cc1 -internal-isystem /vol/worker/clang-debian-cpp20/clang-debian-cpp20/build/lib/clang/20/include -nostdsysteminc -verify -std=c2y -Wall -pedantic -ffreestanding /vol/worker/clang-debian-cpp20/clang-debian-cpp20/llvm-project/clang/test/C/C2y/n3346.c
+ /vol/worker/clang-debian-cpp20/clang-debian-cpp20/build/bin/clang -cc1 -internal-isystem /vol/worker/clang-debian-cpp20/clang-debian-cpp20/build/lib/clang/20/include -nostdsysteminc -verify -std=c2y -Wall -pedantic -ffreestanding /vol/worker/clang-debian-cpp20/clang-debian-cpp20/llvm-project/clang/test/C/C2y/n3346.c
error: 'expected-error' diagnostics expected but not seen: 
  File /vol/worker/clang-debian-cpp20/clang-debian-cpp20/llvm-project/clang/test/C/C2y/n3346.c Line 52: array initializer must be an initializer list
error: 'expected-error' diagnostics seen but not expected: 
  File /vol/worker/clang-debian-cpp20/clang-debian-cpp20/llvm-project/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