Fix equality comparison type issue

- Compiling a function calling Internal.IntrinsicSupport.EqualityComparerHelpers.StructOnlyEquals<T> from a function that is itself generic on T where T is Nullable<SomeGenericStructWhichImplementsIEQuatable<__Canon>> produces an internal compiler error
- The issue is that instantiating a method of Canon isn't actually permitted to be a "real, callable" instantiation
- Fix by changing the "default" variant of StructOnlyEquals to depend on EqualityComparer<T> which is known to work (by using universal generics)
- Then add a variant for just using the Equals function
- Change the compiler to use the "Normal" variant most of the time
- Except for the newly discovered problematic case which can fall back to the slower, but semantically correct EqualityComparar<T> path

[tfs-changeset: 1711845]

src/Common/src/TypeSystem/IL/ILProvider.cs

@@ -225,6 +225,10 @@ namespace Internal.IL
                                 {
                                     methodToCall = helperType.GetKnownMethod("StructOnlyEqualsIEquatable", null).MakeInstantiatedMethod(elementType);
                                 }
+                                else
+                                {
+                                    methodToCall = helperType.GetKnownMethod("StructOnlyNormalEquals", null).MakeInstantiatedMethod(elementType);
+                                }
 
                                 if (methodToCall != null)
                                 {

src/System.Private.CoreLib/src/Internal/IntrinsicSupport/EqualityComparerHelpers.cs

@@ -195,10 +195,15 @@ namespace Internal.IntrinsicSupport
 #endif
         }
 
+        private static bool StructOnlyNormalEquals<T>(T left, T right)
+        {
+            return left.Equals(right);
+        }
+
         [Intrinsic]
         internal static bool StructOnlyEquals<T>(T left, T right)
         {
-            return left.Equals(right);
+            return EqualityComparer<T>.Default.Equals(left, right);
         }
     }
 }