[libcxx] Add "install-libcxx" target.

Summary: Currently you can't install libc++ from within the LLVM tree without installing all of LLVM. This patch adds an install rule for libc++.

Reviewers: mclow.lists, danalbert, jroelofs, EricWF

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D11697

llvm-svn: 245470