[OpenCL] Improve diagnostic for placement new

[svenvh]

Summary

Without an explicit declaration for placement new, clang would reject
uses of placement new with "default new' is not supported in OpenCL
C++", suggesting that placement new is not supported, see e.g.
PR42060.

[Anastasia]

LGTM. Thanks!

include/clang/Basic/DiagnosticSemaKinds.td

 def err_opencl_builtin_expected_type : Error<
   "illegal call to %0, expected %1 argument type">;
 
 // OpenCL v2.2 s2.1.2.3 - Vector Component Access
 def ext_opencl_ext_vector_type_rgba_selector: ExtWarn<
   "vector component name '%0' is an OpenCL version 2.2 feature">,
   InGroup<OpenCLUnsupportedRGBA>;
 
+def err_openclcxx_placement_new : Error<
+  "use of placement new requires explicit declaration">;
+
 // MIG routine annotations.
 def warn_mig_server_routine_does_not_return_kern_return_t : Warning<
   "'mig_server_routine' attribute only applies to routines that return a kern_return_t">,
   InGroup<IgnoredAttributes>;
 } // end of sema category
 
 let CategoryName = "OpenMP Issue" in {
 // OpenMP support.

lib/Sema/SemaExprCXX.cpp

// Find the allocation function.
     if (R.empty()) {
       if (NewScope == AFS_Class)
         return true;
 
       LookupQualifiedName(R, Context.getTranslationUnitDecl());
     }
 
     if (getLangOpts().OpenCLCPlusPlus && R.empty()) {
+      if (PlaceArgs.empty()) {
         Diag(StartLoc, diag::err_openclcxx_not_supported) << "default new";
+      } else {
+        Diag(StartLoc, diag::err_openclcxx_placement_new);
+      }
       return true;
     }
 
     assert(!R.empty() && "implicitly declared allocation functions not found");
     assert(!R.isAmbiguous() && "global allocation functions are ambiguous");
 
     // We do our own custom access checks below.
     R.suppressDiagnostics();

test/SemaOpenCLCXX/newdelete.cl

class B {
   int bx;
 };
 
 // There are no global user-defined new operators at this point. Test that clang
 // rejects these gracefully.
 void test_default_new_delete(void *buffer, A **pa) {
   A *a = new A;         // expected-error {{'default new' is not supported in OpenCL C++}}
   delete a;             // expected-error {{'default delete' is not supported in OpenCL C++}}
-  *pa = new (buffer) A; // expected-error {{'default new' is not supported in OpenCL C++}}
+  *pa = new (buffer) A; // expected-error {{use of placement new requires explicit declaration}}
 }
 
 // expected-note@+1 {{candidate function not viable: requires 2 arguments, but 1 was provided}}
 void *operator new(size_t _s, void *ptr) noexcept {
   return ptr;
 }
 
 // expected-note@+1 {{candidate function not viable: requires 2 arguments, but 1 was provided}}