Fix missing build dependency on omp_gen.

Summary:
`include/llvm/Frontend/OpenMP/CMakeLists.txt` creates a new target
called `omp_gen`, which is automatically added to
`LLVM_COMMON_DEPENDS` by the `add_public_tablegen_target` macro. But
it only gets added to the version of `LLVM_COMMON_DEPENDS` in the
scope of that subsidiary CMakeLists file, and it doesn't propagate all
the way back up to the permanent version of that variable which is
actually used to set dependencies.

The visible effect is that the output build scripts contain a missing
dependency. For example, if I run cmake in Ninja output mode, and then
run

  ninja -t commands tools/clang/examples/PrintFunctionNames/CMakeFiles/PrintFunctionNames.dir/PrintFunctionNames.cpp.o

to list all the commands that are prerequisites of building that
object file, then the list does not include the llvm-tblgen command
that builds `include/llvm/Frontend/OpenMP/OMP.h.inc`, even though that
generated include file is needed (by a chain of includes starting from
`clang/AST/AST.h`), and that object file can't be compiled without it.
This missing dependency can cause intermittent build failures,
depending on the order that Ninja (or whatever) happens to schedule
its commands.

I've fixed it by adding a `set` command in two levels of
`CMakeLists.txt` to propagate the modified version of
`LLVM_COMMON_DEPENDS` back up through the parent scopes so that
`omp_gen` does end up on the version seen by the main
`llvm/CMakeLists.txt`.

Reviewers: clementval, thakis, chandlerc, jdoerfert

Reviewed By: clementval

Subscribers: jdenny, mgorny, sstefan1, llvm-commits

Tags: #llvm

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