[Clang][SemaCXX] Add unused warning for variables declared in condition expressions

This patch marks the declarations with initializations in condition expressions such as
if (int var = init) as unreferenced so that -Wunused can warn on them.

Fixes https://github.com/llvm/llvm-project/issues/61681

Reviewed By: cor3ntin
Differential Revision: https://reviews.llvm.org/D152495