Allow string literals to be cast to mutable

It is legal in constfolding, and it's already impossible
to modify a string literal in CTFE.
CTFE: Fix comparison of string enums

String enum compares was failing to compile because it wasn't
checking the base type, so wasn't recognizing it as a string.
Merge pull request #1804 from donc/string_enum

Fix treatment of string enums in CTFE