IDL compiler: Overloaded methods (initial)

Initial overloaded methods handling.
Overload handling is quite complicated, so I'm breaking up over several CLs.
This does basics, but does not do:
* full resolution handling (just checks length), or
* extended attributes.
Will do these in followups.

Main logic is |compute_overloads| in v8_interface.py;
template code is straightforward.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/68343002

git-svn-id: svn://svn.chromium.org/blink/trunk@161711 bbb929c8-8fbe-4397-9dbb-9b2b20218538