Frontend: Add -enable-parser-lookup flag

This is for re-enabling it once it is turned off by default.

Author: slavapestov

include/swift/Option/Options.td

@@ -1102,7 +1102,11 @@ def scan_clang_dependencies : Flag<["-"], "scan-clang-dependencies">,
 
 def disable_parser_lookup : Flag<["-"], "disable-parser-lookup">,
   Flags<[FrontendOption]>,
-  HelpText<"Disable parser lookup & use ast scope lookup only (experimental)">;
+  HelpText<"Disable parser lookup & use ASTScope lookup only (experimental)">;
+
+def enable_parser_lookup : Flag<["-"], "enable-parser-lookup">,
+  Flags<[FrontendOption]>,
+  HelpText<"Enable parser lookup">;
 
 def enable_request_based_incremental_dependencies : Flag<["-"],
   "enable-request-based-incremental-dependencies">,

lib/Driver/ToolChains.cpp

@@ -269,6 +269,7 @@ void ToolChain::addCommonFrontendArgs(const OutputInfo &OI,
   inputArgs.AddLastArg(arguments, options::OPT_print_educational_notes);
   inputArgs.AddLastArg(arguments, options::OPT_diagnostic_style);
   inputArgs.AddLastArg(arguments, options::OPT_disable_parser_lookup);
+  inputArgs.AddLastArg(arguments, options::OPT_enable_parser_lookup);
   inputArgs.AddLastArg(arguments,
                        options::OPT_enable_experimental_concise_pound_file);
   inputArgs.AddLastArg(

lib/Frontend/CompilerInvocation.cpp

@@ -425,7 +425,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
       = A->getOption().matches(OPT_enable_target_os_checking);
   }
 
-  Opts.DisableParserLookup |= Args.hasArg(OPT_disable_parser_lookup);
+  Opts.DisableParserLookup |= Args.hasFlag(OPT_disable_parser_lookup,
+                                           OPT_enable_parser_lookup,
+                                           /*default*/ false);
   Opts.EnableNewOperatorLookup = Args.hasFlag(OPT_enable_new_operator_lookup,
                                               OPT_disable_new_operator_lookup,
                                               /*default*/ false);