[clang] [clang][Parser] Warn when the body of expansion statement is not a compound statement (PR #209229)

[via cfe-commits]

https://github.com/Sirraide commented:

I’m not sure it makes sense to put this into a new diagnostic group because I don’t think anyone is going to enable this. It might make more sense to put this in `-pedantic`, but also GCC doesn’t warn on this _at all_ (even w/ `-pedantic`). 

I also find it rather strange that the standard even requires a compound-statement here; that just seems like an arbitrary restriction. I wonder if we should file either a core issue or a GCC bug for this?

CC @AaronBallman, @cor3ntin for opinions

https://github.com/llvm/llvm-project/pull/209229