[clang] Check for nullptr when instantiating late attrs

[adamcz]

Summary

This was already done in SemaTemplateInstantiateDecl.cpp, but not in
SemaTemplateInstantiate.cpp.

Anecdotally I've seen some clangd crashes where coredumps point to this
being a problem, but I cannot reproduce this so far.

[hokein]

this looks reasonable to me, thanks for tracking it down!

clang/lib/Sema/SemaTemplateInstantiate.cpp

for (LateInstantiatedAttrVec::iterator I = LateAttrs.begin(),
     NamedDecl *ND = dyn_cast<NamedDecl>(I->NewDecl);
     CXXRecordDecl *ThisContext =
         dyn_cast_or_null<CXXRecordDecl>(ND->getDeclContext());
     CXXThisScopeRAII ThisScope(*this, ThisContext, Qualifiers(),
                                ND && ND->isCXXInstanceMember());
 
     Attr *NewAttr =
       instantiateTemplateAttribute(I->TmplAttr, Context, *this, TemplateArgs);
+    if (NewAttr)
       I->NewDecl->addAttr(NewAttr);
     LocalInstantiationScope::deleteScopes(I->Scope,
                                           Instantiator.getStartingScope());
   }
   Instantiator.disableLateAttributeInstantiation();
   LateAttrs.clear();
 
   ActOnFinishDelayedMemberInitializers(Instantiation);