No longer diagnose (functionally) empty structures under -Wuninitialized

An empty structure in C has no way to be initialized, so triggering a
-Wuninitialized warning for a variable of empty structure type is not
actionable for users. This silences the false positive warning, which
matches the behavior of GCC as well.

We no longer diagnose if the structure has no members, or has only
zero-sized members (unnamed bit-fields, zero-sized bit-fields, empty
structure types).

Fixes: https://github.com/llvm/llvm-project/issues/26842