[diagtree] Use a different color for unimplemented GCC diagnostic flags

instead of the "enabled by default" color.

It may be technically correct to list unimplemented diagnostics as
"enabled by default" but it's quite misleading.

clang/tools/diagtool/DiagnosticNames.h

@@ -76,11 +76,11 @@ namespace diagtool {
         return *this;
       }
 
-      bool operator==(group_iterator &Other) const {
+      bool operator==(const group_iterator &Other) const {
         return CurrentID == Other.CurrentID;
       }
 
-      bool operator!=(group_iterator &Other) const {
+      bool operator!=(const group_iterator &Other) const {
         return CurrentID != Other.CurrentID;
       }
     };

clang/tools/diagtool/TreeView.cpp

@@ -36,6 +36,17 @@ class TreePrinter {
     return Diags.isIgnored(DiagID, SourceLocation());
   }
 
+  static bool unimplemented(const GroupRecord &Group) {
+    if (!Group.diagnostics().empty())
+      return false;
+
+    for (const GroupRecord &GR : Group.subgroups())
+      if (!unimplemented(GR))
+        return false;
+
+    return true;
+  }
+
   static bool enabledByDefault(const GroupRecord &Group) {
     for (const DiagnosticRecord &DR : Group.diagnostics()) {
       if (isIgnored(DR.DiagID))
@@ -53,7 +64,9 @@ class TreePrinter {
   void printGroup(const GroupRecord &Group, unsigned Indent = 0) {
     out.indent(Indent * 2);
 
-    if (enabledByDefault(Group))
+    if (unimplemented(Group))
+      out << Colors::RED;
+    else if (enabledByDefault(Group))
       out << Colors::GREEN;
     else
       out << Colors::YELLOW;
@@ -117,7 +130,9 @@ class TreePrinter {
 
   void showKey() {
     out << '\n' << Colors::GREEN << "GREEN" << Colors::RESET
-        << " = enabled by default\n\n";
+        << " = enabled by default";
+    out << '\n' << Colors::RED << "RED" << Colors::RESET
+        << " = unimplemented (accepted for GCC compatibility)\n\n";
   }
 };