embedded: don't try to specialize vtables of C++ imported reference-counted classes

Fixes a false compiler error

rdar://165209061

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/Utilities/GenericSpecialization.swift

@@ -48,6 +48,9 @@ private struct VTableSpecializer {
     }
 
     let classDecl = classType.nominal! as! ClassDecl
+    if classDecl.isForeign {
+      return
+    }
     guard let origVTable = context.lookupVTable(for: classDecl) else {
       if context.enableWMORequiredDiagnostics {
         context.diagnosticEngine.diagnose(.cannot_specialize_class, classType, at: errorLocation)

test/embedded/cxx-import-reference.swift

@@ -40,3 +40,12 @@ public func test()
   let c =  C.create()
   c.foo()
 }
+
+public func cast<S,D>(_ s:S, to type:D.Type) -> D {
+  return unsafeBitCast(s, to: type.self)
+}
+
+public func testcast(_ provider: AnyObject) -> C {
+  return cast (provider, to: C.self)
+}
+