ILStrip task incorrectly adds a body to extern methods

[lambdageek]

If you run the ILStrip task on an assembly that contains an extern method that has no body or header, ILStrip adds one back.

public class AccessHelper<W>
{
        [UnsafeAccessor(UnsafeAccessorKind.Constructor)]
        public static extern MyList<W> CreateSized(int n);
}

Original IL:

.class public auto ansi beforefieldinit AccessHelper`1<W>
       extends [System.Runtime]System.Object
{
  .method public hidebysig static class MyList`1<!W>
          CreateSized(int32 n) cil managed
  {
    .custom instance void [System.Runtime]System.Runtime.CompilerServices.UnsafeAccessorAttribute::.ctor([System.Runtime]System.Runtime.CompilerServices.UnsafeAccessorKind) = ( 01 00 00 00 00 00 00 00 )
  } // end of method AccessHelper`1::CreateSized

...
}

After ILStrip:

class public auto ansi beforefieldinit AccessHelper`1<W>
       extends [System.Private.CoreLib]System.Object
{
  .method public hidebysig static class MyList`1<!W>
          CreateSized(int32 A_0) cil managed noinlining
  {
    .custom instance void [System.Private.CoreLib]System.Runtime.CompilerServices.UnsafeAccessorAttribute::.ctor([System.Private.CoreLib]System.Runtime.CompilerServices.UnsafeAccessorKind) = ( 01 00 00 00 00 00 00 00 )
    // Code size       1 (0x1)
    .maxstack  8
    IL_0000:  ret
  } // end of method AccessHelper`1::CreateSized

} // end of class AccessHelper`1

This affects UnsafeAccessor support on Apple mobile platforms (both the runtime samples and xamarin-macios use the ILStrip task). related to #89439, #99830

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[lambdageek]

The reason this affects Mono's [UnsafeAccessor] support is because it is cheaper for us to check "does this method have a body" than to check "does this method have a particular custom attribute". Since the majority of methods have bodies and are not unsafe accessors, we use the has-a-body check to quickly bypass the slow path of working with the custom attribute