[clang] fix: compatible C++ empty record with align UB with gcc (PR #72197)

[John McCall via cfe-commits]

rjmccall wrote:

> The code you noted is supposed to handle two cases, neither of which are relevant to your testcase:
> 
> * Darwin-specific calling convention rules.
> * GNU extensions for zero-size structs (which aren't allowed according to either C or C++ standards, but GNU invented a bunch of non-standard rules for them)

To be clear, this isn't just about zero-sized structs

https://github.com/llvm/llvm-project/pull/72197