cmd/gc: fix type checking loop

cmd/gc: fix type checking loop

CL 4313064 fixed its test case but did not address a
general enough problem:

type T1 struct { F *T2 }
type T2 T1
type T3 T2

could still end up copying the definition of T1 for T2
before T1 was done being evaluated, or T3 before T2
was done.

In order to propagate the updates correctly,
record a copy of an incomplete type for re-execution
once the type is completed. Roll back CL 4313064.

Fixes #3709.

R=ken2
CC=golang-dev, lstoakes
https://golang.org/cl/6301059