[clang] [Clang] Fix Sema::checkArgCount for 0-arg functions (PR #139638)

[Hood Chatham via cfe-commits]

hoodmane wrote:

> are we sure they weren't intentionally skipping diagnostic there?

The behavior before was 
```
error: cannot compile this scalar expression yet
    2 |   __externref_t ref = __builtin_wasm_ref_null_extern(0);
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
  ...
```
And there was no test coverage. So I'm pretty confident that it was a mistake. 

https://github.com/llvm/llvm-project/pull/139638