Hoist handling of Doxygen into the top-level cmake/ directory so it can
be shared between LLVM and RUNTIMES and a default/standalone runtimes
build can support building Doxygen documentation as well.
The openmp subproject currently supports doxygen documentation using an
`LLVM_ENABLE_PROJECTS=openmp` build, but not with
`LLVM_ENABLE_RUNTIMES=openmp` because of this missing boilerplate code
in the runtimes build. This is a step towards removing the
`LLVM_ENABLE_PROJECTS=openmp` build mode which was deprecated (#124014)
and already scheduled to be removed in LLVM 21 (#136314). Eventual
removal is planned with #176950.
Hoisting CMake code for shared use with runtimes has been done before in
e.g. #84641, 7017e6c9cfd2de3122ce9528f338a97d61e96373,
44e3365775101fec3fd355eda339282258d74415,
7017e6c9cfd2de3122ce9528f338a97d61e96373
---------
Co-authored-by: Petr Hosek <phosek@google.com>
50638e01e4