[libc][support][FixedVector] add reverse iterator (#86732)

Critically, we don't want to return an iterator to the end of the underlying
cpp::array "store." Add a test to catch this issue.

This will be used by __cxa_finalize to iterate backwards through a FixedVector.

Link: #85651
GitOrigin-RevId: ad97ee25315778a9a25d5b7365308072fbd846d0
2 files changed