Fix _bridgeToObjectiveC lookup code in optimizeBridgedSwiftToObjCCast

Previously it assumed that if we succeed in looking up the method in the current
module we must be able to request a definition (vs a declaration).

This is not true. It could be that we had declared the type in a different
module. Always ask for a declaration.

rdar://27547957

lib/SIL/SILModule.cpp

@@ -316,8 +316,10 @@ SILFunction *SILModule::getOrCreateFunction(SILLocation loc,
 
   if (auto fn = lookUpFunction(name)) {
     assert(fn->getLoweredFunctionType() == constantType);
-    assert(fn->getLinkage() == constant.getLinkage(ForDefinition)
-           || fn->getLinkage() == constant.getLinkage(NotForDefinition));
+    assert(fn->getLinkage() == linkage ||
+           (forDefinition == ForDefinition_t::NotForDefinition &&
+            fn->getLinkage() ==
+                constant.getLinkage(ForDefinition_t::ForDefinition)));
     if (forDefinition) {
       // In all the cases where getConstantLinkage returns something
       // different for ForDefinition, it returns an available-externally