[CMake] Use find_package(LLVM) instead of LLVMConfig

This addresses an issues introduced in r362047.

Differential Revision: https://reviews.llvm.org/D62640

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@362065 91177308-0d34-0410-b5e6-96231b3b80d8