Fix missing build dependency on omp_gen.

Summary:
`include/llvm/Frontend/OpenMP/CMakeLists.txt` creates a new target
called `omp_gen` which builds the generated include file `OMP.h.inc`.
This target must therefore be a dependency of every compilation step
whose transitive #include dependencies contain `OMP.h.inc`, or else
it's possible for builds to fail if Ninja (or make or whatever)
schedules that compilation step before building `OMP.h.inc` at all.

A few of those dependencies are currently missing, which leads to
intermittent build failures, depending on the order that Ninja (or
whatever) happens to schedule its commands. As far as I can see,
compiles in `clang/lib/CodeGen`, `clang/lib/Frontend`, and
`clang/examples` all depend transitivily on `OMP.h.inc` (usually via
`clang/AST/AST.h`), but don't have the formal dependency in the ninja
graph.

Adding `omp_gen` to the dependencies of `clang-tablegen-targets` seems
to be the way to get the missing dependency into the `clang/examples`
subdirectory. This also fixes the other two clang subdirectories, as
far as I can see.

Reviewers: clementval, thakis, chandlerc, jdoerfert

Reviewed By: clementval

Subscribers: cfe-commits, jdenny, mgorny, sstefan1, llvm-commits

Tags: #llvm, #clang

Differential Revision: https://reviews.llvm.org/D82659