[cmake] Fix dependency issue in TableGen

[pzheng]

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.

[aganea]

Makes sense, thank you for fixing this!

[pzheng]

Thanks for reviewing, aganea!

[smeenai]

LGTM, thanks.

cmake/modules/TableGen.cmake

if( ${${project}_TABLEGEN} STREQUAL "${target}" )
       set(${project}_TABLEGEN_EXE ${${project}_TABLEGEN_EXE} PARENT_SCOPE)
 
       add_custom_target(${project}-tablegen-host DEPENDS ${${project}_TABLEGEN_EXE})
       set(${project}_TABLEGEN_TARGET ${project}-tablegen-host PARENT_SCOPE)
 
       # Create an artificial dependency between tablegen projects, because they
       # compile the same dependencies, thus using the same build folders.
       # FIXME: A proper fix requires sequentially chaining tablegens.
-      if (NOT ${project} STREQUAL LLVM AND TARGET ${project}-tablegen-host)
+      if (NOT ${project} STREQUAL LLVM AND TARGET ${project}-tablegen-host AND
+          TARGET LLVM-tablegen-host)
         add_dependencies(${project}-tablegen-host LLVM-tablegen-host)
       endif()
     endif()
   endif()
 
   if (${project} STREQUAL LLVM AND NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
     set(export_to_llvmexports)
     if(${target} IN_LIST LLVM_DISTRIBUTION_COMPONENTS OR