[clang] Add test for CWG255

[Endill]

Summary

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)

[cor3ntin]

LGTM

clang/test/CXX/drs/dr2xx.cpp

 // RUN: %clang_cc1 -std=c++98 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++11 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++14 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++17 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++20 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++2b %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 
 // PR13819 -- __SIZE_TYPE__ is incompatible.
 typedef __SIZE_TYPE__ size_t; // expected-error 0-1 {{extension}}
 
 #if __cplusplus < 201103L
 #define fold(x) (__builtin_constant_p(x) ? (x) : (x))
 #else
 #define fold
template<typename T> struct A {
     typedef struct T::type foo; // expected-error {{typedef 'type' cannot be referenced with a struct specifier}}
   };
   struct B { struct type {}; };
   struct C { typedef struct {} type; }; // expected-note {{here}}
   A<B>::type n;
   A<C>::type n; // expected-note {{instantiation of}}
 }
 
+namespace dr255 { // dr255: yes
+struct S {
+  void operator delete(void *){};
+  void operator delete(void *, int){};
+};
+void f(S *p) { delete p; }
+} // namespace dr255
+
 // dr256: dup 624
 
 namespace dr257 { // dr257: yes
   struct A { A(int); }; // expected-note {{here}}
   struct B : virtual A {
     B() {}
     virtual void f() = 0;
   };

clang/www/cxx_dr_status.html

<tr id="254">
     <td>CD1</td>
     <td>Definitional problems with <I>elaborated-type-specifier</I>s</td>
     <td class="full" align="center">Yes</td>
   </tr>
   <tr id="255">
     <td><a href="https://wg21.link/cwg255">255</a></td>
     <td>CD6</td>
     <td>Placement deallocation functions and lookup ambiguity</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="full" align="center">Yes</td>
   </tr>
   <tr id="256">
     <td><a href="https://wg21.link/cwg256">256</a></td>
     <td>CD1</td>
     <td>Overflow in size calculations</td>
     <td class="none" align="center">Duplicate of <a href="#624">624</a></td>
   </tr>
   <tr id="257">