[libc++] Use a proper CMake target to represent libc++ headers

Instead of having complex logic around how to include the libc++ headers
and __config_site, handle that by defining cxx-headers as an INTERFACE
library and linking against it. After this patch, linking against cxx-headers
is sufficient to get the right __config_site include and include paths
for libc++.

Differential Revision: https://reviews.llvm.org/D82702