go/types: better handle arrays whose length expression is invalid

While doing that, establish a negative value as signal for unknown
array lengths and adjust various array-length processing code to
handle that case.

Fixes #23712.

Change-Id: Icf488faaf972638b42b22d4b4607d1c512c8fc2c
Reviewed-on: https://go-review.googlesource.com/93438


Reviewed-by: Alan Donovan <adonovan@google.com>