IDL compiler: test case + parser fix for [Constructor]

Preliminary CL for constructors:
* adds a test case (there are several related extended attributes to test)
* fixes minor bug in parser:
constructors don't have a return type of their own,
since it's just the interface, so this fixes typedef resolution
to work correctly.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/106193002

git-svn-id: svn://svn.chromium.org/blink/trunk@163237 bbb929c8-8fbe-4397-9dbb-9b2b20218538