cmd/gc: replace "typechecking loop" by nicer errors in some cases.
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