[cxx-interop] Fix nested structs for non-opaque types
The test for nested constructs used library evolution forcing all types
to be opaque. As a result some code paths for non-opaque types were not
updated to support nested types. This patch updates the rest of the code
making sure we use fully qualified names (so they also work in the
context of the nested classes), and generate correct names for the C
compatibility structs that cannot contain "::".
Fixes #80291
rdar://147882976
