[4.0] IRGen: Fix linkage for shared declarations

lib/IRGen/GenDecl.cpp

@@ -1416,7 +1416,8 @@ getIRLinkage(const UniversalLinkageInfo &info, SILLinkage linkage,
 
   case SILLinkage::Shared:
   case SILLinkage::SharedExternal:
-    return RESULT(LinkOnceODR, Hidden, Default);
+    return isDefinition ? RESULT(LinkOnceODR, Hidden, Default)
+                        : RESULT(External, Hidden, Default);
 
   case SILLinkage::Hidden:
     return RESULT(External, Hidden, Default);

test/IRGen/Inputs/usr/include/NSOption.h

@@ -0,0 +1,7 @@
+#define CF_OPTIONS(_type, _name) enum _name : _type _name; enum _name : _type
+#define NS_OPTIONS(_type, _name) CF_OPTIONS(_type, _name)
+
+typedef NS_OPTIONS(int, SomeOptions) {
+  SomeOptionsFoo = 1,
+  SomeOptionsBar = 2,
+};

test/IRGen/objc_shared_imported_decl.sil

@@ -0,0 +1,16 @@
+// RUN: %target-swift-frontend -primary-file %s -import-objc-header %S/Inputs/usr/include/NSOption.h -emit-ir | %FileCheck %s
+
+// REQUIRES: objc_interop
+
+import Swift
+
+sil public @use_witness : $@convention(thin) () -> () {
+  %0 = witness_method $SomeOptions, #Equatable."=="!1 : <Self where Self : Equatable> (Self.Type) -> (Self, Self) -> Bool : $@convention(witness_method) <τ_0_0 where τ_0_0 : Equatable> (@in τ_0_0, @in τ_0_0, @thick τ_0_0.Type) -> Bool
+  %1 = tuple ()
+  return %1: $()
+}
+
+// We used to emit linkonce_odr llvm linkage for this declaration.
+sil_witness_table shared SomeOptions : Equatable module __ObjC
+
+// CHECK: @_T0SC11SomeOptionsVs9EquatableSoWP = external hidden global