[OpenMP] Ensure the DefaultMapperId has a location

[jdoerfert]

Summary

A user reported an assertion (below) but without a reproducer. I failed to
create a test myself but from the assertion one can derive the problem.
I set the DefaultMapperId location now to make sure this doesn't cause
trouble.

clang-13: .../DeclTemplate.h:1940:
void clang::ClassTemplateSpecializationDecl::setPointOfInstantiation(clang::SourceLocation):
Assertion `Loc.isValid() && "point of instantiation must be valid!"' failed.

[JonChesterfield]

Not ideal to have unreproducible failures but the fix looks safe regardless

clang/lib/Sema/SemaOpenMP.cpp

 /// clauses for such members with the found 'default' mapper.
 static void
 processImplicitMapsWithDefaultMappers(Sema &S, DSAStackTy *Stack,
                                       SmallVectorImpl<OMPClause *> &Clauses) {
   // Check for the deault mapper for data members.
   if (S.getLangOpts().OpenMP < 50)
     return;
   SmallVector<OMPClause *, 4> ImplicitMaps;
-  DeclarationNameInfo DefaultMapperId;
-  DefaultMapperId.setName(S.Context.DeclarationNames.getIdentifier(
-      &S.Context.Idents.get("default")));
   for (int Cnt = 0, EndCnt = Clauses.size(); Cnt < EndCnt; ++Cnt) {
     auto *C = dyn_cast<OMPMapClause>(Clauses[Cnt]);
     if (!C)
       continue;
     SmallVector<Expr *, 4> SubExprs;
     auto *MI = C->mapperlist_begin();
     for (auto I = C->varlist_begin(), End = C->varlist_end(); I != End;
          ++I, ++MI) {
for (auto I = C->varlist_begin(), End = C->varlist_end(); I != End;
         // Only structs/classes are allowed to have mappers.
         const RecordDecl *RD = BaseType.getCanonicalType()->getAsRecordDecl();
         if (!RD)
           continue;
         auto It = Visited.find(BaseType.getTypePtr());
         if (It == Visited.end()) {
           // Try to find the associated user-defined mapper.
           CXXScopeSpec MapperIdScopeSpec;
+          DeclarationNameInfo DefaultMapperId;
+          DefaultMapperId.setName(S.Context.DeclarationNames.getIdentifier(
+                                  &S.Context.Idents.get("default")));
+          DefaultMapperId.setLoc(E->getExprLoc());
           ExprResult ER = buildUserDefinedMapperRef(
               S, Stack->getCurScope(), MapperIdScopeSpec, DefaultMapperId,
               BaseType, /*UnresolvedMapper=*/nullptr);
           if (ER.isInvalid())
             continue;
           It = Visited.try_emplace(BaseType.getTypePtr(), ER.get()).first;
         }
         // Found default mapper.
         if (It->second) {
           auto *OE = new (S.Context) OpaqueValueExpr(E->getExprLoc(), CanonType,
                                                      VK_LValue, OK_Ordinary, E);
           OE->setIsUnique(/*V=*/true);