Add new flag for ByRefLike constraints (#945)

* Add new flag for ByRefLike constraints

* Allow seems to be the prefered nomenclature

Author: jbevain

Mono.Cecil/GenericParameter.cs

@@ -165,6 +165,11 @@ public bool HasDefaultConstructorConstraint {
 			set { attributes = attributes.SetAttributes ((ushort) GenericParameterAttributes.DefaultConstructorConstraint, value); }
 		}
 
+		public bool AllowByRefLikeConstraint {
+			get { return attributes.GetAttributes ((ushort) GenericParameterAttributes.AllowByRefLikeConstraint); }
+			set { attributes = attributes.SetAttributes ((ushort) GenericParameterAttributes.AllowByRefLikeConstraint, value); }
+		}
+
 		#endregion
 
 		public GenericParameter (IGenericParameterProvider owner)

Mono.Cecil/GenericParameterAttributes.cs

@@ -22,6 +22,7 @@ public enum GenericParameterAttributes : ushort {
 		SpecialConstraintMask			= 0x001c,
 		ReferenceTypeConstraint			= 0x0004,
 		NotNullableValueTypeConstraint	= 0x0008,
-		DefaultConstructorConstraint	= 0x0010
+		DefaultConstructorConstraint	= 0x0010,
+		AllowByRefLikeConstraint = 0x0020,
 	}
 }