go/types: less closure creations in gcimporter.

Closures are incredibly expensive on linux/arm due to
repetitive flush of instruction cache.

go test -short on ODROID-X:

Before:
ok      exp/gotype      17.091s
ok      go/types        2.225s

After:
ok      exp/gotype      7.193s
ok      go/types        1.143s

R=dave, minux.ma, rsc
CC=golang-dev, remy
https://codereview.appspot.com/7062045