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