[Diagnostics] Use the new swift-syntax formatter for -diagnostic-style swift.

We're going to move toward the new swift-syntax formatter. Use it for
the "swift" diagnostic style instead of the experimental formatter
written in C++.

There are some tests for the experimental formatter in C++ that test
precise formatting of diagnostics. I've disabled them in the same
places where the new swift-syntax formatter is enabled, for now. We
may choose to remove them entirely, because swift-syntax itself is
where the specific format is defined, and has those same kinds of
tests already.

lib/Frontend/CompilerInvocation.cpp

@@ -1517,9 +1517,6 @@ static bool ParseDiagnosticArgs(DiagnosticOptions &Opts, ArgList &Args,
       Opts.PrintedFormattingStyle = DiagnosticOptions::FormattingStyle::LLVM;
     } else if (contents == "swift") {
       Opts.PrintedFormattingStyle = DiagnosticOptions::FormattingStyle::Swift;
-    } else if (contents == "swift-syntax") {
-      Opts.PrintedFormattingStyle =
-          DiagnosticOptions::FormattingStyle::SwiftSyntax;
     } else {
       Diags.diagnose(SourceLoc(), diag::error_unsupported_option_argument,
                      arg->getOption().getPrefixedName(), arg->getValue());