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

[Gábor Horváth via cfe-commits]

https://github.com/Xazax-hun commented:

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. 

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