[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.

(cherry picked from commit e239ee8)

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