IDL compiler: [ImplementedAs] on interface type

This adds support for [ImplementedAs] on an interface type, i.e., when
[ImplementedAs] is in *a different* IDL file.

Now that interfaces_info is in place, this is easy, mostly plumbing,
and *much* faster than the Perl way (which parses IDL files for all
interface types that are used!).

BUG=239771
R=haraken

Review URL: https://codereview.chromium.org/123873003

git-svn-id: svn://svn.chromium.org/blink/trunk@164679 bbb929c8-8fbe-4397-9dbb-9b2b20218538