IRGen: Don't emit reflection metadata for builtin types unless we're building the standard library

I couldn't reduce a test case, but it seems to come up with optimizations
enabled. We should have already emitted metadata for everything when we
built the stdlib, no need to do it again.

Fixes <rdar://problem/28924516>.

Author: slavapestov

lib/IRGen/GenReflection.cpp

@@ -180,7 +180,7 @@ class ReflectionMetadataBuilder : public ConstantBuilder<> {
   // Collect any builtin types referenced from this type.
   void addBuiltinTypeRefs(CanType type) {
     type.visit([&](Type t) {
-      if (t->is<BuiltinType>())
+      if (IGM.getSwiftModule()->isStdlibModule() && t->is<BuiltinType>())
         IGM.BuiltinTypes.insert(CanType(t));
 
       // We need size/alignment information for imported value types,