[clang] [C] Disallow declarations where a statement is required (PR #92908)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

I'm less certain, but CC @jdoerfert @mikerice1969 for more opinions.

The original changes actually fixed a bug in OpenMP related to standalone directives.

> Note, this seems to have fixed an issue with some OpenMP stand-alone
directives not being properly diagnosed as per:
https://www.openmp.org/spec-html/5.1/openmpsu19.html#x34-330002.1.3
(The same requirement exists in OpenMP 5.2 as well.)

https://github.com/llvm/llvm-project/pull/92908