[libc++] Improve the format ignorelist generation.

Several improvements
- Only add files that we actually want to format.
- Sort according to a fixed locale.

Some drive-by fixes
- Rename a text file, this avoids a filter exception.
- Adds a some missing source files extensions.
- Removes unused extensions hh, hxx, cc, and cxx from clang-format.

Reviewed By: philnik, #libc

Differential Revision: https://reviews.llvm.org/D144126