[clang][X86] psABI diagnostics are not emitted when no FunctionDecl is available

We have psABI diagnostics implemented but they only work when there is a valid FunctionDecl. We should expand this to also cover cases where no FunctionDecl is available.

For example gcc emits a warning here but clang does not:

```c
typedef short avx256Type __attribute__((vector_size(32)));
typedef void (*fp_arg_t)(avx256Type);

__attribute__((used)) static void call_fp_arg(fp_arg_t fp) {
avx256Type x = (avx256Type){0};
fp(x); // Missing -Wpsabi warning.
}
```

https://godbolt.org/z/6xG9EETqa

For context see discussion in https://github.com/llvm/llvm-project/pull/199091

Start by reproducing the shown C example with Clang and inspect the existing psABI diagnostic handling, especially how it depends on a FunctionDecl. Use the linked discussion for context. Done means the no-FunctionDecl case emits the expected -Wpsabi warning, with coverage for the example.

Written by the indexing model from the issue text.