Fix compile issue in generated bitset XXChoiceValues struct

[zhangw]

Before the fix, the generated bitset-related code looked like this:

type PresenceMapChoiceValue uint8
type PresenceMapChoiceValues struct {
FooField PresenceMapChoiceValue
BarField PresenceMapChoiceValue
}
var PresenceMapChoice = PresenceMapChoiceValues{uint64(0), uint64(1)}

This caused a compile error:
cannot use uint64(0) (constant 0 of type uint64) as PresenceMapChoiceValue value in struct literal
compilerIncompatibleAssign
The issue is that uint64(0) cannot be directly assigned to a field of type PresenceMapChoiceValue (uint8 alias).

After the fix, the generated code is:
var PresenceMapChoice = PresenceMapChoiceValues{0, 1}

This is simple, correct, and compiles without the issues before.

[vyazelenko]

@zhangw Please add a test for this fix.