[ELF] Parallelize writes of different OutputSections

We currently process one OutputSection at a time and for each OutputSection
write contained input sections in parallel. This strategy does not leverage
multi-threading well. Instead, parallelize writes of different OutputSections.

The default TaskSize for parallelFor often leads to inferior sharding. We
prepare the task in the caller instead.

* Move llvm::parallel::detail::TaskGroup to llvm::parallel::TaskGroup
* Add llvm::parallel::TaskGroup::execute.
* Change writeSections to declare TaskGroup and pass it to writeTo.

Speed-up with --threads=8:

* clang -DCMAKE_BUILD_TYPE=Release: 1.11x as fast
* clang -DCMAKE_BUILD_TYPE=Debug: 1.10x as fast
* chrome -DCMAKE_BUILD_TYPE=Release: 1.04x as fast
* scylladb build/release: 1.09x as fast

On M1, many benchmarks are a small fraction of a percentage faster. Mozilla showed the largest difference with the patch being about 1.03x as fast.

Differential Revision: https://reviews.llvm.org/D131247