Sema: Go back to synthesizing hashValue on _StoredBridgedNSError conformers

Commit 22294f2 added a default implementation, however this is wrong
for non-imported types.

Instead, synthesize the body as before. Since this is one of the few
derived methods that can appear on an imported type, make sure to
build fully type-checked AST.

Fixes <rdar://problem/51322302>.

Author: slavapestov

Darwin/Foundation-swiftoverlay/NSError.swift

@@ -489,10 +489,6 @@ extension _BridgedStoredNSError {
   public func hash(into hasher: inout Hasher) {
     hasher.combine(_nsError)
   }
-
-  @_alwaysEmitIntoClient public var hashValue: Int {
-    return _nsError.hashValue
-  }
 }
 
 /// Describes the code of an error.