Weekday is redundant information for a Time structure.
When parsing a time with a weekday specified, it can create an
incorrect Time value.
When parsing a time without a weekday specified, people
expect the weekday to be set.
Fix all three problems by computing the weekday on demand.

This is hard to gofix, since we must change the type of the node.
Since uses are rare and existing code will be caught by the compiler,
there is no gofix module here.

Fixes #2245.

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/4974077