[CMake] Replace use of llvm-config with LLVM and Clang CMake packages

Summary:
I did this for two reasons:
- Using the CMake packages simplifies building LLDB Standalone. This is for two
  reasons: 1) We were doing a decent amount of work that is already done in the
  LLVMConfig.cmake that we want to import, 2) We had to do some manual work to call
  llvm-config, parse its output, and populate variables that the build system
  uses.
- As far as I understand, using llvm-config makes it difficult if not impossible
  to cross-compile LLDB standalone.

Reviewers: sgraenitz, labath, zturner, JDevlieghere, davide, aprantl, stella.stamenova

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D56531

llvm-svn: 351863