cmd/cgo: build unique C type cache keys from parent names

When translating C types, cache the in-progress type under its parent
names, so that anonymous structs can also be translated for multiple
typedefs, without clashing.

Standalone types are not affected by this change.

Also updated the test for issue 9026 because the C struct name
generation algorithm has changed.

Fixes #31891

Change-Id: I00cc64852a2617ce33da13f74caec886af05b9f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/181857


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