[clang] [Clang] Don't treat a declaration that declares nothing as an invalid statement (PR #224682)

[Ambrose Leeb via cfe-commits]

Sirraide wrote:

> so `if (x) int; foo();` would take `foo();` as the substatement.

To clarify, with this change, the `foo()` is now the body of the `if`? Because, that would be very wrong: `int;` is still a statement that we accept.

https://github.com/llvm/llvm-project/pull/224682