CMake: Modernize Clang/LLVM find scripts
Changes:
- LLVM_INCLUDE_DIR -> LLVM_INCLUDE_DIRS
- LLVM_LIBRARY_DIR -> LLVM_LIBRARY_DIRS
- CLANG_INCLUDE_DIR -> CLANG_INCLUDE_DIRS
- CLANG_LIBRARY_DIR -> CLANG_LIBRARY_DIRS
- Introduce LLVM_INSTALL_PREFIX
- Fix compilation with self-built non-installed LLVM/Clang