[clang] Add test for CWG255

[[https://wg21.link/p1787 | P1787]]: CWG255 (partially resolved by N3778 is resolved by generally specifying the restriction to usual deallocation functions.
Wording: In any case, any declarations other than of usual deallocation functions ([basic.stc.dynamic.deallocation]) are discarded. ([expr.delete]/9)

Reviewed By: #clang-language-wg, cor3ntin

Differential Revision: https://reviews.llvm.org/D147549