We do not uniqueify cxx::bridge names except for functions

This error encountered on an internal codebase:

`"thread '' panicked at 'Unable to generate header and C++ code: Syn(Error("the name `Key` is defined multiple times"))', /src/main.rs:189:18"`

Working on reducing now.

Start at /src/main.rs:189:18 and reproduce the reported generation failure using the reduced internal case. Trace how cxx::bridge names are handled, with the duplicate `Key` definition as the test case; the work is done when distinct bridge names no longer trigger this panic.

Written by the indexing model from the issue text.