[CMake] Add llvm-tblgen to dependencies of check-llvm.

llvm-tblgen is not built when external LLVM_TABLEGEN is specified.
Even then, llvm-tblgen should be built for testing tblgen itself.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199558 91177308-0d34-0410-b5e6-96231b3b80d8