Partial extern members missing warning about DllImport

[jjonescz]

Version Used: main

Steps to Reproduce:

partial class C
{
    public partial void M1();
    public extern partial void M1();

    public extern void M2();
}

Expected Behavior: M1 and M2 to get the same warnings since they are equivalent.

Actual Behavior: Only M2 gets this warning:

// /TestProject/Program.cs(6,24): warning CS0626: Method, operator, or accessor 'C.M2()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
//     public extern void M2();
Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M2").WithArguments("C.M2()").WithLocation(6, 24)

Notes: See c2c76c9 where I started fixing this for partial events and constructors but reverted it since it seems orthogonal to the feature.

[jaredpar]

@RikkiGibson is this a scenario that we missed when expanding partial members? Or do we have a defined behavior for extern partial?

[RikkiGibson]

I think we just missed it. I think a regular extern T M(); which is missing a DllImport is going to do the same thing as an extern partial T M(); missing the same.

[jjonescz]

Yeah, this behavior is for partial methods as well.

Also note that this is a slight breaking change because it adds a warning where it wasn't previously.