[OPENMP] Fix incomplete type check for array reductions

[jdenny]

Summary

A reduction for an incomplete array type used to produce an assert
fail during codegen. Now it produces a diagnostic.

[ABataev]

LG

clang/lib/Sema/SemaOpenMP.cpp

     } else {
       Type = Context.getBaseElementType(D->getType().getNonReferenceType());
     }
     auto *VD = dyn_cast<VarDecl>(D);
 
     // OpenMP [2.9.3.3, Restrictions, C/C++, p.3]
     //  A variable that appears in a private clause must not have an incomplete
     //  type or a reference type.
-    if (S.RequireCompleteType(ELoc, Type,
+    if (S.RequireCompleteType(ELoc, D->getType(),
                               diag::err_omp_reduction_incomplete_type))
       continue;
     // OpenMP [2.14.3.6, reduction clause, Restrictions]
     // A list item that appears in a reduction clause must not be
     // const-qualified.
     if (Type.getNonReferenceType().isConstant(Context)) {
       S.Diag(ELoc, diag::err_omp_const_reduction_list_item) << ERange;
       if (!ASE && !OASE) {

clang/test/OpenMP/parallel_reduction_messages.c

+// RUN: %clang_cc1 -verify -fopenmp -ferror-limit 150 -o - %s
+
+int incomplete[];
+
+void test() {
+#pragma omp parallel reduction(+ : incomplete) // expected-error {{a reduction list item with incomplete type 'int []'}}
+  ;
+}
+
+// complete to suppress an additional warning, but it's too late for pragmas
+int incomplete[3];