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.)

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@189128 91177308-0d34-0410-b5e6-96231b3b80d8