Add a -gnone option that can negate a previous -g option.

The name -gnone was chosen by analogy with -O and -Onone. Like -O/-Onone,
the last option on the command line wins.

The immediate use case for this is because we want to be able to run the
tests with -g injected into every command line, but some tests will fail
when debug info is included. Those particular tests can be explicitly marked
-gnone.

rdar://problem/18636307

Swift SVN r22777

lib/Frontend/CompilerInvocation.cpp

@@ -892,14 +892,19 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
                            StringRef ResourceDir) {
   using namespace options;
 
-  if (Args.hasArg(OPT_g)) {
-    Opts.DebugInfo = true;
-    ArgStringList RenderedArgs;
-    for (auto A : Args)
-      A->render(Args, RenderedArgs);
-    CompilerInvocation::buildDWARFDebugFlags(Opts.DWARFDebugFlags,
-                                             RenderedArgs, SDKPath,
-                                             ResourceDir);
+  if (const Arg *A = Args.getLastArg(OPT_g_Group)) {
+    if (A->getOption().matches(OPT_g)) {
+      Opts.DebugInfo = true;
+      ArgStringList RenderedArgs;
+      for (auto A : Args)
+        A->render(Args, RenderedArgs);
+      CompilerInvocation::buildDWARFDebugFlags(Opts.DWARFDebugFlags,
+                                               RenderedArgs, SDKPath,
+                                               ResourceDir);
+    } else {
+      assert(A->getOption().matches(options::OPT_gnone) &&
+             "unknown -g<kind> option");
+    }
   }
 
   for (const Arg *A : make_range(Args.filtered_begin(OPT_l, OPT_framework),