Force color diagnostics when using the driver from the command line.

Previously, the frontend detected that its output was being piped into the
driver and buffered, and decided that that wasn't a color-friendly output
stream. Now, the driver passes -color-diagnostics to the frontend to force
color output if the driver itself is in a color-output context.

<rdar://problem/16697713>

Swift SVN r18506

lib/Frontend/CompilerInvocation.cpp

@@ -645,6 +645,7 @@ static bool ParseDiagnosticArgs(DiagnosticOptions &Opts, ArgList &Args,
   Opts.SkipDiagnosticPasses |= Args.hasArg(OPT_disable_diagnostic_passes);
   Opts.ShowDiagnosticsAfterFatalError |=
     Args.hasArg(OPT_show_diagnostics_after_fatal);
+  Opts.UseColor |= Args.hasArg(OPT_color_diagnostics);
 
   return false;
 }