cmake: llvm-config related enhancement
cmake: llvm-config related enhancement

* fail if LLVM_DIR is specified incorrectly
* fail fast if llvm-config is not found
* print LLVM_DIR

iamrekcah/clay,mario-campos/clay,jckarter/clay,iamrekcah/clay,mario-campos/clay,jckarter/clay,jckarter/clay,mario-campos/clay,iamrekcah/clay,iamrekcah/clay,mario-campos/clay,iamrekcah/clay,jckarter/clay,mario-campos/clay,jckarter/clay,jckarter/clay