DebugInfo: Accept -gdwarf even in clang-cl

Fixes regression introduced by llvmorg-10-init-8908-g098d901bd1b

Author: dwblaikie

clang/include/clang/Driver/Options.td

@@ -1974,7 +1974,7 @@ def glldb : Flag<["-"], "glldb">, Group<gTune_Group>;
 def gsce : Flag<["-"], "gsce">, Group<gTune_Group>;
 // Equivalent to our default dwarf version. Forces usual dwarf emission when
 // CodeView is enabled.
-def gdwarf : Flag<["-"], "gdwarf">, Group<g_Group>,
+def gdwarf : Flag<["-"], "gdwarf">, Group<g_Group>, Flags<[CoreOption]>,
   HelpText<"Generate source-level debug information with the default dwarf version">;
 def gdwarf_2 : Flag<["-"], "gdwarf-2">, Group<g_Group>,
   HelpText<"Generate source-level debug information with dwarf version 2">;