[Frontend] Separate parse-only functionality into its own function, CompilerInstance::performParseOnly().

Parse-only is a hot path; keep the semantics for it separate from normal parsing, otherwise it is very
easy to introduce something expensive without checking for Invocation.getParseOnly().

Also cleans up a bit CompilerInstance::performParse() as well.

Swift SVN r17596

lib/Frontend/CompilerInvocation.cpp

@@ -159,9 +159,6 @@ static bool ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
       Action = FrontendOptions::Parse;
     } else if (Opt.matches(OPT_dump_parse)) {
       Action = FrontendOptions::DumpParse;
-
-      // -dump-parse explicitly disables type-checking
-      Opts.ParseOnly = true;
     } else if (Opt.matches(OPT_dump_ast)) {
       Action = FrontendOptions::DumpAST;
     } else if (Opt.matches(OPT_print_ast)) {