[clang] Guard type size queries on incomplete types in -Wunsafe-buffer-usage (PR #205426)

[Yitzhak Mandelbaum via cfe-commits]

ymand wrote:

> The code changes overall look good to me, but the test only seems to cover the incomplete type scenario, not the dependent type one. I think it would be great to cover both to make sure future changes would not break either of those.

+1 Good catch, I missed that in my pre-review. 

https://github.com/llvm/llvm-project/pull/205426