[libc++] Don't commit libcxx.imp (#89391)

We can instead generate it on-the-fly when we install the headers. This
reduces the amount of boilerplate we have to re-generate whenever we
add, remove or relocate header files.

Fixes #88529

GitOrigin-RevId: e2754890ca75d64b3acb9b8655874421aed8f9cb
4 files changed