Report some cases where a method decorated with [UnsafeAccessorAttribute] is not valid.
Note: Because some references doesn't expose their private members through Roslyn, it's not possible to validate the full signature.
[UnsafeAccessor(UnsafeAccessorKind.Field, Name = "dummy")]
extern static ref int Demo(MyStruct a); // Not compliant as the first parameter is not by ref
[UnsafeAccessor(UnsafeAccessorKind.Field, Name = "dummy")]
extern static ref int Demo(ref MyStruct a); // ok