IDL compiler: Overloaded methods: nullable interface types

This adds the minor existing type-checking for nullable interface type
to overloaded methods.
Turns out there are no nullable arrays in actual IDLs,
so I haven't implemented those, and I've removed the test case.

It also does:
* some other cleaning up of overloading test cases;
* adds a FIXME comment to the Python parser about nullable arrays
  and arrays of nullables (Foo[]? and Foo?[]).
  (Noticed when trying tests.)
  Neither is used, but we don't handle these or other complex types
  correctly (sequence of arrays etc.), as they make the code much
  more complicated and are not used, but are legal Web IDL.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/71063002

git-svn-id: svn://svn.chromium.org/blink/trunk@161883 bbb929c8-8fbe-4397-9dbb-9b2b20218538