Recommit r243503 "[libcxx] Cleanup CMake configuration and integrate with LLVM"

This change was reverted in r243550 because it broke clang-format builds
(see PR24306).

This patch recommits a fixed version of the original.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@243574 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed