[Clang][Sema] Warn unused cxx vardecl which entirely consists condition expr...

[Clang][Sema] Warn unused cxx vardecl which entirely consists condition expr of if/while/for construct (#87348)

Emit `-Wunused-but-set-variable` warning on C++ variables whose
declaration (with initializer) entirely consist the condition expression
of a if/while/for construct but are not actually used in the body of the
if/while/for construct.

Fixes #41447