Reapply working parts of CMake cleanup.

This patch adds the working parts of r243503. The difference with this patch
is that it doesn't include the HandleLLVMOptions.cmake file.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@243698 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed