[clang] Emit an error if variable ends up with incomplete array type

This adds an error if variable with incomplete type has initializer with
incomplete type, so it is not possible to deduce array size from
initializer.

Fixes https://github.com/llvm/llvm-project/issues/37257

Reviewed By: aaron.ballman, shafik

Differential Revision: https://reviews.llvm.org/D158615