For issue 3757:
BEFORE:  test/fixedbugs/bug463.go:12: typechecking loop involving a
             test/fixedbugs/bug463.go:12 a
             test/fixedbugs/bug463.go:12 <node DCLCONST>
AFTER:   test/fixedbugs/bug463.go:12: constant definition loop
             test/fixedbugs/bug463.go:12: a uses a

For issue 3937:
BEFORE: test/fixedbugs/bug464.go:12: typechecking loop involving foo
            test/fixedbugs/bug464.go:12 <T>
            test/fixedbugs/bug464.go:12 foo
            test/fixedbugs/bug464.go:12 <node DCLFUNC>
AFTER:  test/fixedbugs/bug464.go:12: foo is not a type

Fixes #3757.
Fixes #3937.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6614058