C#13: Add the "allow byref-like" anti-constraint

[Zastai]

This adds a new AllowsByRefLike flag to GenericParameterAttributes which, unlike the others, is an anti-constraint (i.e. it removes restrictions rather than adding them) added in C# 13. Because of this, AllowsByRefLike was chosen as the corresponding bool property on GenericParameter (instead of
HasAllowByRefLineConstraint; the constant also does not include Constraint in its name).

At the same time, it adds the NoSpecialConstraint flag and a corresponding HasNoSpecialConstraint property.

Reference: corresponding constants in the CLR code: https://github.com/dotnet/runtime/blob/9daa4b41eb9f157e79eaf05e2f7451c9c8f6dbdc/src/coreclr/inc/corhdr.h#L850

[Zastai]

Hmm, looks like it's already there; could have sworn I updated my fork.

[Zastai]

Right, still keeping this PR because:

• it updates SpecialConstraintMask to include the new flag
• I kept the enum constant name in like with the CLR/COR header (no Constraint at the end, because it's not a constraint)
• the property name lacked the Has (needed because the Allow referred only to the ByRefLike, not the Constraint
  • I'm still keeping AllowsByRefLike for now, because it reads better than HasAllowByRefLike
    • but I'm open to changing it to the latter for consistency with the other property names
• it also adds the NoSpecialConstraint flag and property, to better match the whole of the CLR/COR version of the attributes