Previously, on encountering Func.Nname.Type == nil, typecheckfunc()
returned without initializing Decldepth for that func. This causes
typecheckclosure() to fatal. This change ensures that we initialize
Decldepth in all cases.

Fixes #17588.

Change-Id: I2e3c81ad52e8383395025388989e8dbf03438b68
Reviewed-on: https://go-review.googlesource.com/32415Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>