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

tools/swift-syntax-test/swift-syntax-test.cpp

@@ -69,7 +69,7 @@ int getTokensFromFile(unsigned BufferID,
                       LangOptions &LangOpts,
                       SourceManager &SourceMgr,
                       DiagnosticEngine &Diags,
-                      std::vector<std::pair<syntax::RC<syntax::TokenSyntax>,
+                      std::vector<std::pair<RC<syntax::TokenSyntax>,
                       syntax::AbsolutePosition>> &Tokens) {
   Tokens = tokenizeWithTrivia(LangOpts, SourceMgr, BufferID);
   return Diags.hadAnyError() ? EXIT_FAILURE : EXIT_SUCCESS;
@@ -81,7 +81,7 @@ getTokensFromFile(const StringRef InputFilename,
                   LangOptions &LangOpts,
                   SourceManager &SourceMgr,
                   DiagnosticEngine &Diags,
-                  std::vector<std::pair<syntax::RC<syntax::TokenSyntax>,
+                  std::vector<std::pair<RC<syntax::TokenSyntax>,
                                         syntax::AbsolutePosition>> &Tokens) {
   auto Buffer = llvm::MemoryBuffer::getFile(InputFilename);
   if (!Buffer) {
@@ -101,7 +101,7 @@ int doFullLexRoundTrip(const StringRef InputFilename) {
   PrintingDiagnosticConsumer DiagPrinter;
   Diags.addConsumer(DiagPrinter);
 
-  std::vector<std::pair<syntax::RC<syntax::TokenSyntax>,
+  std::vector<std::pair<RC<syntax::TokenSyntax>,
                                    syntax::AbsolutePosition>> Tokens;
   if (getTokensFromFile(InputFilename, LangOpts, SourceMgr,
                         Diags, Tokens) == EXIT_FAILURE) {
@@ -123,7 +123,7 @@ int doDumpTokenSyntax(const StringRef InputFilename) {
   Diags.addConsumer(DiagPrinter);
 
 
-  std::vector<std::pair<syntax::RC<syntax::TokenSyntax>,
+  std::vector<std::pair<RC<syntax::TokenSyntax>,
                         syntax::AbsolutePosition>> Tokens;
   if (getTokensFromFile(InputFilename, LangOpts, SourceMgr,
                         Diags, Tokens) == EXIT_FAILURE) {
@@ -172,7 +172,7 @@ int doFullParseRoundTrip(const StringRef InputFilename) {
   assert(SF && "No source file");
 
   // Retokenize the buffer with full fidelity
-  std::vector<std::pair<syntax::RC<syntax::TokenSyntax>,
+  std::vector<std::pair<RC<syntax::TokenSyntax>,
                         syntax::AbsolutePosition>> Tokens;
   if (getTokensFromFile(BufferID, Invocation.getLangOptions(),
                         SourceMgr,