Fix crash with parsing stdarg.h (or around this on Mac). Simple skip fake va_list_tag_name declaration as in clang repo in lib/Sema/SemaLookup.cpp
Separated IsTagDecl into TryGetClass and TryGetEnum, added IsClass() and IsEnum().
Merge pull request #239 from vovkasm/fix_crash_with_va_list_tag

Fix crash with parsing stdarg.h (or around this on Mac).
Merge pull request #241 from esdrubal/TryGetClass

Separated IsTagDecl into TryGetClass and TryGetEnum, added IsClass() and...
Replaced some IsGenerated by IsInternal.
CheckIgnoreField now properly handles internal fields.
Declaration.IsInternal is no longer true when Declaration.GenerationKind is Generate.