[clang] [clang] Improve diagnostics with incompatible VLA types (PR #101261)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

It looks like precommit CI found some issues: https://buildkite.com/llvm-project/github-pull-requests/builds/93641#0191755a-81be-4291-9346-95bf02e2b39b
```
Clang :: Sema/array-init.c
Clang :: Sema/gnu-flags.c
```

https://github.com/llvm/llvm-project/pull/101261