SILGen: Minimal workaround for multiple-emission bug with Clang-importer-synthesized properties on enums.

If you extend a C/ObjC enum type to conform to ErrorType, then the synthesized _code getter ends up registered as an external decl, but appears to already get synthesized due to @slavapestov's changes. I'm not sure whether this is reliable enough that we can simply stop registering the external decls, so just to keep the build from crashing, insert a check in emitExternalDecls that we didn't already emit the decl and skip emission. Low-risk workaround for rdar://problem/24287125.

Author: jckarter

lib/SILGen/SILGenDecl.cpp

@@ -1134,6 +1134,14 @@ void SILGenModule::emitExternalWitnessTable(ProtocolConformance *c) {
 void SILGenModule::emitExternalDefinition(Decl *d) {
   switch (d->getKind()) {
   case DeclKind::Func: {
+    auto fd = cast<FuncDecl>(d);
+    // FIXME: rdar://problem/24287125 causes us to emit some property accessors
+    // more than once. As a minimal workaround, skip emitting the function if
+    // the SILFunction for it already exists.
+    if (auto existing = getEmittedFunction(SILDeclRef(fd), NotForDefinition))
+      if (!existing->empty())
+        break;
+    
     emitFunction(cast<FuncDecl>(d));
     break;
   }

test/SILGen/Inputs/objc_enum_errortype.h

@@ -0,0 +1,3 @@
+enum AXError {
+    kAXErrorSuccess = 0,
+};

test/SILGen/objc_enum_errortype.swift

@@ -0,0 +1,7 @@
+// RUN: %target-swift-frontend -emit-silgen %s -import-objc-header %S/Inputs/objc_enum_errortype.h | FileCheck %s
+
+// REQUIRES: objc_interop
+
+// CHECK: sil @_TFE19objc_enum_errortypeVSC7AXErrorg5_codeSi
+
+extension AXError: ErrorType { }