[openmp] Build doxygen in bootstrapping builds (#178298)
When LLVM_ENABLE_DOXYGEN=ON, forward the `doxygen-openmp` build target
from the nested (default target) runtimes build. When
LLVM_BUILD_DOCS=ON, also trigger `doxygen-build` with `ninja doxygen`.
LLVM_INCLUDE_DOCS=ON is required in the runtimes build, which is the
default.
This is required to update the OpenMP doxygen documentation at
https://openmp.llvm.org/doxygen by the publish-doxygen-docs buidbot,
discussed here:
https://github.com/llvm/llvm-zorg/pull/716#pullrequestreview-3713032311
