[cmake] Fix dependency issue in TableGen
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):
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
Differential Revision: https://reviews.llvm.org/D60576