Clean up enum raw expression validation a bit
The distinction between the type checked raw value expression and the regular raw value expression was never important. Downstream clients were ignoring the type checked form and pulling the text out of the supposed "plain" form. Drop the distinction and simply don't set back into the raw value expr if we don't have to.
Pushing this through naturally enables some cleanup in checkEnumRawValues. Factor out type checking the literal value into an helper on the typechecker and pull a common diagnostic into the decl checker.
