[libc++][docs] Overhaul the documentation for building and using libc++

This patch overhauls the documentation around building libc++
for vendors, and using libc++ for end-users. It also:

- Removes mention of the standalone build, which we've been trying to
  get rid of for a long time.
- Removes mention of using a local ABI installation, which we don't do
  and is documented as "not recommended".
- Removes mention of the separate libc++filesystem.a library, which isn't
  relevant anymore since filesystem support is in the main library.
- Adds mention of the GDB pretty printers and how to use them.