vhpi: Support EnumLiterals

vhpi: Support EnumLiterals

Add support for the various *Literals properties of enums. Although
`EnumLiteral`s are generalizations of both `expr`s and `decl`s, we only
really support single-inheritance at the moment. Make c_enumLiteral a
subtype of c_abstractDecl, since there are more properties in `decl`s
than in `expr`s. This should be the only exception in this area we need
to make, since other literals aren't generalizations of decl.