ChiselEnum: cannot use .asUInt.litValue to get the litValue

**Type of issue**: bug report / feature request

**Impact**: API addition (no impact on existing code)

**Development Phase**: request

**Other information**:

When using `ChiselEnum`, one should be able to take enum literals and use `asUInt` followed by `litValue`. This pattern is notably useful when trying to turn enum literals into `BitPat`s.

```scala
object ExampleEnum extends ChiselEnum {
val Foo, Bar = Value
}

// Does not work since BitPat() internally calls .litValue which crashes with None.get
val simple = BitPat(ExampleEnum.Foo.asUInt)

// Instead we have to use a clumsy workaround
val complicated = BitPat(ExampleEnum.Foo.litValue.U(ExampleEnum.getWidth.W))
```

Start by locating the ChiselEnum, asUInt, and litValue entry points, then trace how BitPat(ExampleEnum.Foo.asUInt) reaches litValue. Reproduce the None.get failure and verify that enum literals passed through asUInt expose the expected literal value without breaking the existing workaround.

Written by the indexing model from the issue text.