[clang] better error message for while loops outside of control flow

report an error when encountering 'while' token parsing declarator

```
clang/test/Parser/while-loop-outside-function.c:3:1: error: while loop outside of a function
while // expected-error {{while loop outside of a function}}
^
clang/test/Parser/while-loop-outside-function.c:7:1: error: while loop outside of a function
while // expected-error {{while loop outside of a function}}
^
```

Fixes: https://github.com/llvm/llvm-project/issues/34462

Differential Revision: https://reviews.llvm.org/D129573