IDL compiler: [Constructor] overloading

This completes [Constructor].

Relatively long, b/c need to support list of constructors, but most of
logic already handled in method overloading.

A few notes:
* Also simplify Perl logic
* Some stub values (e.g., argument['is_nullable'] = False)
  b/c needed by overload resolution algorithm.
* moved constructor macro into methods.cpp b/c otherwise cyclic import
  between interface_base.cpp and interface.cpp.
* interface.cpp diff looks weird b/c it replaces the actual constructor
  method with the overload callback.

BUG=239771

Review URL: https://codereview.chromium.org/112303003

git-svn-id: svn://svn.chromium.org/blink/trunk@163939 bbb929c8-8fbe-4397-9dbb-9b2b20218538