swift-module-digester: refactor the ABI/API diagnostics to use DiagnosticEngine. NFC (#18957)

Before this patch, we used to define tooling-specific diagnostics. With adding more
checking logics, we found this mechanism hard to extend. This patch
eliminates the home-made diagnostics model to use the one from compiler, which provides
several benefits: less boiler-templates, better integration with DiagnosticConsumer,
and easy ordering of detected issues.

include/swift/AST/DiagnosticsModuleDiffer.h

@@ -0,0 +1,32 @@
+//===--- DiagnosticsModuleDiffer.h - Diagnostic Definitions ----*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+/// \file
+/// \brief This file defines diagnostics for the Swift module differ.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_DIAGNOSTICS_MODULE_DIFFER_H
+#define SWIFT_DIAGNOSTICS_MODULE_DIFFER_H
+
+#include "swift/AST/DiagnosticsCommon.h"
+
+namespace swift {
+  namespace diag {
+  // Declare common diagnostics objects with their appropriate types.
+#define DIAG(KIND,ID,Options,Text,Signature) \
+  extern detail::DiagWithArguments<void Signature>::type ID;
+#include "DiagnosticsModuleDiffer.def"
+  }
+}
+
+#endif