cmd/cgo: don't resolve typedefs in -godefs mode

cmd/cgo: don't resolve typedefs in -godefs mode

In -godefs mode any typedefs that appear in struct fields and the like
will presumably be defined in the input file. If we resolve to the
base type, those cross-references will not work. So for -godefs mode,
keep the Go 1.10 behavior and don't resolve the typedefs in a loop.

Fixes #26644

Change-Id: I48cf72d9eb5016353c43074e6aff6495af326f35
Reviewed-on: https://go-review.googlesource.com/125995
Reviewed-by: Keith Randall <khr@golang.org>