[cmake] Fix dependency issue in TableGen

Summary:
There is a bug in add_tablegen which causes cmake to fail with the following
error message if LLVM_TABLEGEN is set.

CMake Error at cmake/modules/TableGen.cmake:147 (add_dependencies):
  The dependency target "LLVM-tablegen-host" of target "CLANG-tablegen-host"
  does not exist.
Call Stack (most recent call first):
  tools/clang/utils/TableGen/CMakeLists.txt:3 (add_tablegen)

The issue happens because setting LLVM_TABLEGEN causes cmake to skip generating
the LLVM-tablegen-host target. As a result, a non-existent target was added for
CLANG-tablegen-host causing cmake to fail.

In order to fix this issue, this patch adds a guard to check the validity of the
dependency target before adding it as a dependency.

Reviewers: aganea, smeenai

Reviewed By: aganea

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60576

llvm-svn: 358226