[libcxx] Cleanup CMake configuration and integrate with LLVM

[libcxx] Cleanup CMake configuration and integrate with LLVM

Summary:
This patch contains the following changes:

1. Require that libc++ can find a LLVM source directory. This is done the same way as `libc++abi` currently does.
2. Cleanup ugly configuration code in CMakeLists.txt by using `add_flags`, `add_flags_if`, and `add_flags_if_supported` macros.

The goals for this patch are:

1. Help libc++ be more consistent with how LLVM handles CMake options (see PR23670 PR23671).
2. Make it easier to use sanitizers using the `LLVM_USE_SANITIZER` option.
3. Make libc++'s CMakeLists.txt file easier to understand and change.
4. Move towards allowing libc++ to create Sphinx documentation (see http://efcs.ca/libcxx-docs).
5. Move towards allowing  libc++ to use other LLVM utilities such as `not` and `FileCheck`.

Reviewers: mclow.lists, jroelofs, danalbert

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D11308

llvm-svn: 243503