Reflection: Allow ParameterBuilder.SetConstant(nonNullValue) for nullable enum parame...

Reflection: Allow ParameterBuilder.SetConstant(nonNullValue) for nullable enum parameters (dotnet/coreclr#17977)

* ParameterBuilder.SetConstant(nonNull) for TEnum?

This change makes it possible to use `ParameterBuilder.SetConstant`
to set a non-null default value for parameters having a nullable enum
type.

* ParameterInfo.DefaultValue for TEnum?

Add a note to `MdConstant.GetValue` (which sits behind `ParameterInfo.
[Raw]DefaultValue`) explaining why changing its behavior to mirror the
change in `TypeBuilder.SetConstantValue` would be a breaking change.

Commit migrated from https://github.com/dotnet/coreclr/commit/f1c47fa7d2cdf29c5d0d1d7c89855e0d0ee5518a