Update the docs for building libc++.

Summary:
Rewrite the in-tree build to be a clearer tl;dr like we have for the
out-of-tree build.

Reviewers: EricWF, mclow.lists, ldionne

Reviewed By: ldionne

Subscribers: dexonsmith, christof, ldionne, enh, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D69917