[NFC] Rewrite and restructure diagnostics handling in FrontendTool

Extract the logics for emitting diagnostics, especially the parseable
output message, from FrontendTools so it is easier to reason.

Author: cachemeifyoucan

include/swift/Frontend/DiagnosticHelper.h

@@ -0,0 +1,65 @@
+//===--- DiagnosticHelper.h - Diagnostic Helper -----------------*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2020 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This file exposes helper class to emit diagnostics from swift-frontend.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_FRONTEND_DIAGNOSTIC_HELPER_H
+#define SWIFT_FRONTEND_DIAGNOSTIC_HELPER_H
+
+#include "swift/Basic/LLVM.h"
+
+namespace swift {
+class CompilerInstance;
+class CompilerInvocation;
+
+class DiagnosticHelper {
+private:
+  class Implementation;
+  Implementation &Impl;
+
+public:
+  /// Create a DiagnosticHelper class to emit diagnostics from frontend actions.
+  static DiagnosticHelper create(CompilerInstance &instance,
+                                 bool useQuasiPID = false);
+
+  /// Initialized all DiagConsumers and add to the CompilerInstance.
+  void initDiagConsumers(CompilerInvocation &invocation);
+
+  /// Begin emitting the message, specifically the parseable output message.
+  void beginMessage(CompilerInvocation &invocation,
+                    ArrayRef<const char *> args);
+
+  /// End emitting all diagnostics. This has to be called if beginMessage() is
+  /// called.
+  void endMessage(int retCode);
+
+  /// Set if printing output should be suppressed.
+  void setSuppressOutput(bool suppressOutput);
+
+  /// Helper function to emit fatal error.
+  void diagnoseFatalError(const char *reason, bool shouldCrash);
+
+  DiagnosticHelper(const DiagnosticHelper &) = delete;
+  DiagnosticHelper(DiagnosticHelper &&) = delete;
+  DiagnosticHelper &operator=(const DiagnosticHelper &) = delete;
+  DiagnosticHelper &operator=(DiagnosticHelper &&) = delete;
+  ~DiagnosticHelper();
+
+private:
+  DiagnosticHelper(CompilerInstance &instance, bool useQuasiPID);
+};
+
+} // namespace swift
+
+#endif

lib/Frontend/CMakeLists.txt

@@ -10,6 +10,7 @@ add_swift_host_library(swiftFrontend STATIC
   CompileJobCacheResult.cpp
   CompilerInvocation.cpp
   DependencyVerifier.cpp
+  DiagnosticHelper.cpp
   DiagnosticVerifier.cpp
   Frontend.cpp
   FrontendInputsAndOutputs.cpp