cmd/cgo, misc/cgo: only cache anonymous struct typedefs with parent name

CL 181857 broke the translation of certain C types using cmd/cgo -godefs
because it stores each typedef, array and qualified type with their
parent type name in the translation cache.

Fix this by only considering the parent type for typedefs of anonymous
structs which is the only case where types might become ambiguous.

Updates #31891
Fixes #37479
Fixes #37621

Change-Id: I301a749ec89585789cb0d213593bb8b7341beb88
Reviewed-on: https://go-review.googlesource.com/c/go/+/226341


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>