Remove `toEnumVariantName` from `RustSymbolProvider` (#2377)

The `toEnumVariantName` function existed on symbol provider to work
around enum definitions not being shapes. In the future when we refactor
to use `EnumShape` instead of `EnumTrait`, there will be `MemberShape`s
for each enum member. This change incrementally moves us to that future
by creating fake `MemberShape`s in the enum generator from the enum
definition.