[generator] Qualify object.GetType() invocations

[jonpryor]

Context: https://bugzilla.xamarin.com/show_bug.cgi?id=45203
Context: https://bugzilla.xamarin.com/show_bug.cgi?id=44263#c8

(Sorry; private bugs.)

The getType() method isn't special; any Java library could contain
such a method:

// Java
public class Example {
    public int[] getType() {return null;}
}

(It's a wonder that we haven't hit such a thing before!)

Unfortunately, should such a method (1) exist, and (2) be bound as
GetType() instead of as a Type property -- in the above example,
we use an int[] return type because if a Java method has an array
return type, we won't turn it into a property -- the resulting code
wouldn't compile:

// error CS0019: Operator `==' cannot be applied to operands of type `int[]' and `System.Type'
if (GetType () == ThresholdType) ...

To fix this, qualify all use of the GetType() method so that we
explicitly use System.Object.GetType():

if ((object) this).GetType () == ThresholdType) ...

This has no performance-impact, IL-wise, as the C# compiler is able to
statically determine that we want non-virtual invocation of the
System.Object.GetType() method. No runtime cast is performed.

[atsushieno]

No, we have been seeing this quite a lot. It should be renamed by metadata fixup.

[atsushieno]

By dealing this as a "supported" form, any future changes to generator will have to be taken care very carefully to not cause this kind of issue which prevents productive developments.

An alternative approach is to detect "getType()" and raise an error.

[jonpryor]

No, we have been seeing this quite a lot.

That suggests that we should do this fix.

It should be renamed by metadata fixup.

Why?

One of the (unobtainable?) goals for generator is that it always work: we give it a .jar file, and we get an assembly, no user intervention required. The API might not be good, the naming might be terrible, but ideally, we'd at least generate something, without requiring that the developer deal with a million errors.

We're clearly far from this goal. Checking for getType() and erroring would get us further from this goal.

By dealing this as a "supported" form, any future changes to generator will have to be taken care very carefully to not cause this kind of issue which prevents productive developments.

Once a test is in place -- and this PR includes a test -- I don't see why we would need to be "very careful" when using GetType() in the future. Additionally, the "alternative" to GetType() is trivial -- ((object) this).GetType().

[atsushieno]

Okay okay, I'm not going to take care of any bugs that is related to this issue, so this can go in.