[serialization] Handle computed properties with a getter /and/ setter.

Since accessor FuncDecl have a backreference to the VarDecl they are
attached to, we were getting re-entrant deserialization issues. Fix
this by just not serializing the backreference and relying on the
property being deserialized first.

Swift SVN r6134

lib/Serialization/ModuleFormat.h

@@ -435,7 +435,7 @@ namespace decls_block {
     BCFixed<1>,   // class method?
     BCFixed<1>,   // assignment?
     TypeIDField,  // type (signature)
-    DeclIDField,  // associated decl (for get/set or operators)
+    DeclIDField,  // operator decl
     DeclIDField   // overridden function
     // The record is trailed by its generic parameters, if any, followed by its
     // argument and body parameter patterns.