[SILGen] Remove an obsolete linking hack

We already make sure @objc entry-points aren't
optimized out.

Author: hamishknight

lib/SIL/IR/SILDeclRef.cpp

@@ -233,13 +233,11 @@ SILLinkage SILDeclRef::getLinkage(ForDefinition_t forDefinition) const {
     return forDefinition ? linkage : addExternalToLinkage(linkage);
   };
 
-  // Native function-local declarations have shared linkage.
-  // FIXME: @objc declarations should be too, but we currently have no way
-  // of marking them "used" other than making them external.
+  // Function-local declarations have private linkage, unless serialized.
   ValueDecl *d = getDecl();
   DeclContext *moduleContext = d->getDeclContext();
   while (!moduleContext->isModuleScopeContext()) {
-    if (!isForeign && moduleContext->isLocalContext()) {
+    if (moduleContext->isLocalContext()) {
       return isSerialized() ? SILLinkage::Shared : SILLinkage::Private;
     }
     moduleContext = moduleContext->getParent();

test/IRGen/objc_local.swift

@@ -1,6 +1,7 @@
 // RUN: %empty-directory(%t)
 // RUN: %build-irgen-test-overlays
 // RUN: %target-swift-frontend(mock-sdk: -sdk %S/Inputs -I %t) %s -emit-ir | %FileCheck %s
+// RUN: %target-swift-frontend(mock-sdk: -sdk %S/Inputs -I %t) %s -emit-ir -O | %FileCheck %s
 
 // REQUIRES: objc_interop