Do not pass null attributes to BuildAttributedStmt during instantiation

When transforming an attribute during template instantiation, if the
transformation fails, it may result in a null attribute being returned.
This null attribute should not be passed in as one of the attributes
used to create an attributed statement.

If all of the attributes fail to transform, we do not create an
attributed statement at all.

There are no attributes that return null currently, so there is no easy
way to test this currently. However, this fixes a crash caused by
8344675.

Author: AaronBallman

clang/lib/Sema/TreeTransform.h

@@ -7325,7 +7325,8 @@ TreeTransform<Derived>::TransformAttributedStmt(AttributedStmt *S,
   for (const auto *I : S->getAttrs()) {
     const Attr *R = getDerived().TransformAttr(I);
     AttrsChanged |= (I != R);
-    Attrs.push_back(R);
+    if (R)
+      Attrs.push_back(R);
   }
 
   StmtResult SubStmt = getDerived().TransformStmt(S->getSubStmt(), SDK);
@@ -7335,6 +7336,11 @@ TreeTransform<Derived>::TransformAttributedStmt(AttributedStmt *S,
   if (SubStmt.get() == S->getSubStmt() && !AttrsChanged)
     return S;
 
+  // If transforming the attributes failed for all of the attributes in the
+  // statement, don't make an AttributedStmt without attributes.
+  if (Attrs.empty())
+    return SubStmt;
+
   return getDerived().RebuildAttributedStmt(S->getAttrLoc(), Attrs,
                                             SubStmt.get());
 }