Update libc++ docs to include instructions for LIT.

Okay, so this actually does more than just that. I've rearranged most of
the information on the page to try to make it more helpful and flow
better.  Essentially, the differences between Mac and Linux, the various
ABI libraries, and in-tree versus out-of-tree builds were cluttering
things. To clean up, I've done the following:

 * Only describe the cmake process. buildit doesn't work out of the box
   on Linux, and we need to stop having duplicates for every process.
 * Use libc++abi for the default instructions. This works on the major
   platforms.
 * Describe both in-tree and out-of-tree builds. Previously it wasn't
   clear that in-tree builds were even possible for libc++.
 * Separate the documentation about using libc++ from that about
   building and testing libc++.