consider reversing the meaning of `hasClassConstraint`

[mayoff]

The hasClassConstraint property of ProtocolDescriptor.Flags returns true if the corresponding bit is set in the protocol descriptor.

However, when the bit is set, it means that the protocol is not class-constrained. Here's how GenMeta.cpp sets the bit:

      flags.setClassConstraint(Proto->requiresClass()
                                 ? ProtocolClassConstraint::Class
                                 : ProtocolClassConstraint::Any);

And here's how MetadataValues.h defines ProtocolClassConstraint:

/// Flag that indicates whether an existential type is class-constrained or not.
enum class ProtocolClassConstraint : bool {
  /// The protocol is class-constrained, so only class types can conform to it.
  ///
  /// This must be 0 for ABI compatibility with Objective-C protocol_t records.
  Class = false,
  /// Any type can conform to the protocol.
  Any = true,
};

Given this, hasClassConstraint would be more meaningful if it returned true when the corresponding bit is not set.