[cmake] Add missing dependencies to objlib in add_llvm_executable.

After f06abbb3 I have been seeing build
failures due to the obj.clang target missing a dependency on
tools/clang/clang-tablegen-targets.

This appears to be due to the fact that LLVM_COMMON_DEPENDS are not added
as dependencies to the object library.

This patch uses the same logic as llvm_add_library to register
dependencies for object libraries.

Reviewed By: beanz, abrachet, steven_wu

Differential Revision: https://reviews.llvm.org/D127318