Sema: Restored AST verification but loosened the check

The check now passes if the type meets isExistentialType rather than isAnyExistentialType.

Author: sl

lib/AST/ASTVerifier.cpp

@@ -1816,6 +1816,15 @@ class Verifier : public ASTWalker {
         abort();
       }
 
+      // The base of a member reference cannot be an existential type.
+      if (E->getBase()->getType()->getWithoutSpecifierType()
+            ->isExistentialType()) {
+        Out << "Member reference into an unopened existential type\n";
+        E->dump(Out);
+        Out << "\n";
+        abort();
+      }
+
       // The only time the base is allowed to be inout is if we are accessing
       // a computed property or if the base is a protocol or existential.
       if (auto *baseIOT = E->getBase()->getType()->getAs<InOutType>()) {