Ensure `GetCorElementType` returns value type for enums on Mono runtime (#72685)

Ensure `GetCorElementType` returns value type for enums on Mono runtime (#72685)

`Type.GetTypeCode` on Mono was calling `GetCorElementType`, which, when invoked on an enum nested in a generic class, would return `TypeCode.Object` instead of `TypeCode.Int32`. Matching to CoreClr, we change `GetCorElementType` to return `VALUETYPE` for enums to fix the issue. The original issue would also provoke Enum parsing to trigger an assert in a debug runtime and go through a slower, rare path for parsing as well.

Fixes https://github.com/dotnet/runtime/issues/72543