Binding projects should be able to bind system frameworks

[chamons]

In #15485 it was found that a customer did this:

  <ItemGroup>
	<ObjcBindingApiDefinition Include="ApiDefinition.cs" />
	<ObjcBindingCoreSource Include="StructsAndEnums.cs" />
	<NativeReference Include="/System/Library/Frameworks/ScriptingBridge.framework" Kind="Framework" />
  </ItemGroup>

while trying to work around #15489.

We should warn or error when the native reference starts with "/System/Library/Frameworks/" as nothing good can come of this.

We should make it possible to bind system frameworks.

[chamons]

FYI @rolfbjarne

[rolfbjarne]

Alternatively we can try to make binding a system framework work, which might be the better option.

[rolfbjarne]

See #16070 for an example where binding a system framework makes sense.