Use find_library for ncurses

Currently it is hard to avoid having LLVM link to the system install of
ncurses, since it uses check_library_exists to find e.g. libtinfo and
not find_library or find_package.

With this change the ncurses lib is found with find_library, which also
considers CMAKE_PREFIX_PATH. This solves an issue for the spack package
manager, where we want to use the zlib installed by spack, and spack
provides the CMAKE_PREFIX_PATH for it.

This is a similar change as https://reviews.llvm.org/D79219, which just
landed in master.

Differential revision: https://reviews.llvm.org/D85820