[CMake] Add missing find_package for LibEdit

After building and installing LLVM with LibEdit as a dependency, it is
necessary to find it again when LLVM is consumed by another CMake
project, otherwise CMake will report an error about a missing target.

Note that the FindLibEdit.cmake file is in the "LLVM Common CMake
Utils" directory, outside of the LLVM sub-project source directory, so
the installed LLVMConfig.cmake relies on the user having installed the
LLVM common CMake modules or make available their own Find module.

Also note that the controlling HAVE_LIBEDIT CMake variable in
LLVMConfig.cmake.in has a different naming convention compared to
other similar variables like 'LLVM_ENABLE_TERMINFO'. Refactoring this
name would involve touching additional files and should be a follow-up
commit.

Patch By: ekilmer

Differential Revision: https://reviews.llvm.org/D147153