This reverts commit a01307a6ee788fc6ac2e09e58f0f52e5666def86 and its
follow-up fix 32d5221ec4810dd723ccebaabbda1df5d3b4cfcf.
It caused unexpected warnings emitted for nested designators in C.
Normally warning is not reported when a field has default initializer.
Do so for anonymous unions with default initializers as well. No release
note since it is a regression in clang 18.
Fixes https://github.com/llvm/llvm-project/issues/70384
91088978d712cd7b33610c59f69d87d5a39e3113 shouldn't have removed an
additional check that field has incomplete array type.
Fixes https://github.com/llvm/llvm-project/issues/66300
Co-authored-by: Aaron Ballman <aaron@aaronballman.com>
