[CMake] If you're not installing the libcxx library, exclude it from the "all"...

[CMake] If you're not installing the libcxx library, exclude it from the "all" target so it doesn't get built when you run "ninja install"

This is just a build dependency optimization. Running check-libcxx will still build libcxx and function as expected, it just removes libcxx from the all build and install targets.

llvm-svn: 254628