Fix warnings when `-Wdeprecated-enum-enum-conversion` is enabled

clang may throw the following warning:
include/clang/AST/DeclarationName.h:210:52: error: arithmetic between
different enumeration types ('clang::DeclarationName::StoredNameKind'
and 'clang::detail::DeclarationNameExtra::ExtraKind') is deprecated
when flags -Werror,-Wdeprecated-enum-enum-conversion are on.

This adds the `addEnumValues()` helper function to STLExtras.h to hide
the details of adding enumeration values together from two different
enumerations.