Start the Migrator library

The Swift 4 Migrator is invoked through either the driver and frontend
with the -update-code flag.

The basic pipeline in the frontend is:

- Perform some list of syntactic fixes (there are currently none).
- Perform N rounds of sema fix-its on the primary input file, currently
  set to 7 based on prior migrator seasons.  Right now, this is just set
  to take any fix-it suggested by the compiler.
- Emit a replacement map file, a JSON file describing replacements to a
  file that Xcode knows how to understand.

Currently, the Migrator maintains a history of migration states along
the way for debugging purposes.

- Add -emit-remap frontend option
  This will indicate the EmitRemap frontend action.
- Don't fork to a separte swift-update binary.
  This is going to be a mode of the compiler, invoked by the same flags.
- Add -disable-migrator-fixits option
  Useful for debugging, this skips the phase in the Migrator that
  automatically applies fix-its suggested by the compiler.
- Add -emit-migrated-file-path option
  This is used for testing/debugging scenarios. This takes the final
  migration state's output text and writes it to the file specified
  by this option.
- Add -dump-migration-states-dir

  This dumps all of the migration states encountered during a migration
  run for a file to the given directory. For example, the compiler
  fix-it migration pass dumps the input file, the output file, and the
  remap file between the two.

  State output has the following naming convention:
  ${Index}-${MigrationPassName}-${What}.${extension}, such as:
  1-FixitMigrationState-Input.swift

rdar://problem/30926261

lib/Frontend/CompilerInvocation.cpp

@@ -323,6 +323,8 @@ static bool ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
       Action = FrontendOptions::REPL;
     } else if (Opt.matches(OPT_interpret)) {
       Action = FrontendOptions::Immediate;
+    } else if(Opt.matches(OPT_update_code)) {
+      Action = FrontendOptions::UpdateCode;
     } else {
       llvm_unreachable("Unhandled mode option");
     }
@@ -560,6 +562,10 @@ static bool ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
         Suffix = "importedmodules";
       break;
 
+    case FrontendOptions::UpdateCode:
+      Suffix = REMAP_EXTENSION;
+      break;
+
     case FrontendOptions::EmitTBD:
       if (Opts.OutputFilenames.empty())
         Opts.setSingleOutputFilename("-");
@@ -706,6 +712,7 @@ static bool ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
     case FrontendOptions::DumpTypeRefinementContexts:
     case FrontendOptions::Immediate:
     case FrontendOptions::REPL:
+    case FrontendOptions::UpdateCode:
       Diags.diagnose(SourceLoc(), diag::error_mode_cannot_emit_dependencies);
       return true;
     case FrontendOptions::Parse:
@@ -738,6 +745,7 @@ static bool ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
     case FrontendOptions::DumpTypeRefinementContexts:
     case FrontendOptions::Immediate:
     case FrontendOptions::REPL:
+    case FrontendOptions::UpdateCode:
       Diags.diagnose(SourceLoc(), diag::error_mode_cannot_emit_header);
       return true;
     case FrontendOptions::Parse:
@@ -773,6 +781,7 @@ static bool ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
     case FrontendOptions::EmitSILGen:
     case FrontendOptions::Immediate:
     case FrontendOptions::REPL:
+    case FrontendOptions::UpdateCode:
       if (!Opts.ModuleOutputPath.empty())
         Diags.diagnose(SourceLoc(), diag::error_mode_cannot_emit_module);
       else
@@ -1535,6 +1544,27 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
   return false;
 }
 
+bool ParseMigratorArgs(MigratorOptions &Opts, ArgList &Args,
+                       DiagnosticEngine &Diags) {
+  using namespace options;
+
+  Opts.AddObjC |= Args.hasArg(OPT_warn_swift3_objc_inference);
+
+  if (Args.hasArg(OPT_disable_migrator_fixits)) {
+    Opts.EnableMigratorFixits = false;
+  }
+
+  if (auto MigratedFilePath = Args.getLastArg(OPT_emit_migrated_file_path)) {
+    Opts.EmitMigratedFilePath = MigratedFilePath->getValue();
+  }
+
+  if (auto Dumpster = Args.getLastArg(OPT_dump_migration_states_dir)) {
+    Opts.DumpMigrationStatesDir = Dumpster->getValue();
+  }
+
+  return false;
+}
+
 bool CompilerInvocation::parseArgs(ArrayRef<const char *> Args,
                                    DiagnosticEngine &Diags,
                                    StringRef workingDirectory) {
@@ -1597,6 +1627,10 @@ bool CompilerInvocation::parseArgs(ArrayRef<const char *> Args,
     return true;
   }
 
+  if (ParseMigratorArgs(MigratorOpts, ParsedArgs, Diags)) {
+    return true;
+  }
+
   updateRuntimeLibraryPath(SearchPathOpts, LangOpts.Target);
 
   return false;