Expose multi-line function argument type annotation comments

[AWhetter]

Feature Request

Say we are parsing the following function:

def sum(
        a,  # type: int
        b,  # type: int
):
    # type: (...) -> int
    return a + b

There is no way to access the type annotation comments for the individual a and b arguments, only the function itself.
typed_ast exposes these type comments through the attribute type_comment on each arg node:

>>> import typed_ast
>>> module = typed_ast.ast3.parse('''def sum(\n    a,  # type: int\n    b,  # type: int\n    ):\n    # type: (...) -> int\n    return a + b''')
>>> module.body[0].args.args
[<typed_ast._ast3.arg object at 0x7f318d7cd0b8>, <typed_ast._ast3.arg object at 0x7f318d7cd0f0>]
>>> module.body[0].args.args[0].type_comment
'int'

Desired behavior

We could add a type_annotation attribute to the AssignName node, but for regular assignments this is redundant information because type comments are exposed on the parent Assign node.
So the parent Arguments node seems like the right place for it to go. I'm proposing the addition of a type_comment_args attribute on the Arguments node that contains a list of the type comments, one for each argument using a None for arguments that are missing a comment.

python -c "from astroid import __pkginfo__; print(__pkginfo__.version)" output

2.2.5

[PCManticore]

This sounds like a sensible suggestion, makes sense having it.