CMake: Don't look for llvm-tblgen when building outside LLVM tree

Previously, the CMake build would look for llvm-tblgen to determine
if a directory is an LLVM build or install directory. Since we don't
want to include llvm-tblgen in the install, look for llvm-config instead,
and use that to find llvm-tblgen.

(This is the lld equivalent of Clang r189127.)