Issue 12385 - Enum member should not be modifiable when the member is immutable.

https://issues.dlang.org/show_bug.cgi?id=12385

@9rnsr: I don't think this is a reliable fix, but I'm not really sure how to proceed. But `TypeEnum`'s mod is always `0`, so I had to look up the EnumDeclaration's memtype's modifier instead.