Add support for generic attributes (dotnet/linker#2980)

* Add generic attribute tests
* Update cecil

Commit migrated from dotnet/linker@e2dba52

src/tools/illink/test/Mono.Linker.Tests.Cases/Attributes/GenericAttributes.cs

@@ -0,0 +1,69 @@
+using System;
+using Mono.Linker.Tests.Cases.Expectations.Assertions;
+using Mono.Linker.Tests.Cases.Expectations.Metadata;
+
+namespace Mono.Linker.Tests.Cases.Attributes
+{
+	class GenericAttributes
+	{
+        static void Main ()
+        {
+            new WithGenericAttribute_OfString ();
+            new WithGenericAttribute_OfInt ();
+            new WithConstrainedGenericAttribute ();
+        }
+
+        [Kept]
+        [KeptAttributeAttribute (typeof (GenericAttribute<string>))]
+        [KeptMember (".ctor()")]
+        [GenericAttribute<string>("t", F = "f", P = "p")]
+        class WithGenericAttribute_OfString {
+        }
+
+        [Kept]
+        [KeptAttributeAttribute (typeof (GenericAttribute<int>))]
+        [KeptMember (".ctor()")]
+        [GenericAttribute<int>(1, F = 2, P = 3)]
+        class WithGenericAttribute_OfInt {
+        }
+
+        [Kept]
+        [KeptAttributeAttribute (typeof (ConstrainedGenericAttribute<DerivedFromConstraintType>))]
+        [KeptMember (".ctor()")]
+        [ConstrainedGenericAttribute<DerivedFromConstraintType>()]
+        class WithConstrainedGenericAttribute {
+        }
+
+        [KeptBaseType (typeof (Attribute))]
+        class GenericAttribute<T> : Attribute {
+            [Kept]
+            public GenericAttribute(T t) {}
+
+            [Kept]
+            public T F;
+
+            [Kept]
+            [KeptBackingField]
+            public T P {
+                get;
+                [Kept]
+                set;
+            }
+        }
+
+        [Kept]
+        class ConstraintType {
+        }
+
+        [KeptBaseType (typeof (ConstraintType))]
+        class DerivedFromConstraintType : ConstraintType {
+        }
+
+        [KeptBaseType (typeof (Attribute))]
+        class ConstrainedGenericAttribute<T> : Attribute
+            where T : ConstraintType {
+            [Kept]
+            public ConstrainedGenericAttribute() {}
+        }
+    }
+}