gc: correctly handle fields of pointer type to recursive forward references

gc: correctly handle fields of pointer type to recursive forward references

Previously, whether declaring a type which copied the structure of a type it was referenced in via a pointer field would work depended on whether you declared it before or after the type it copied, e.g. type T2 T1; type T1 struct { F *T2 } would work, however type T1 struct { F *T2 }; type T2 T1 wouldn't.

Fixes #667.

R=rsc
CC=golang-dev
https://golang.org/cl/4313064