Refactor overloaded methods grouping

Refactoring, no changes to generated code.
This is a technical CL;
the key point is to separate grouping of overloaded methods
from computation of resolution.
(To implement the overload resolution algorithm, we need to first
 have the set of all methods with the same name.)

This also lets us simplify the code;
also factored out some utility functions,
so now main grouping function is quite simple.

R=haraken
BUG=293561

Review URL: https://codereview.chromium.org/273573003

git-svn-id: svn://svn.chromium.org/blink/trunk@173492 bbb929c8-8fbe-4397-9dbb-9b2b20218538