[clang] Report missing designated initializers in C++

Prior to this change clang didn't emit missing-field-initializers
warning for designated initializers. The comments say that it is done to
match gcc behavior. However, gcc behaves so only for C. For C++ warnings
are emitted.

Fixes https://github.com/llvm/llvm-project/issues/56628

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D157879