[pstl] Grand Central Dispatch PSTL backend patch ready for review

https://reviews.llvm.org/D120186

Sorry for the delay on this. Also, I’m not 100% sure I was able to test it. I ran:

cmake -G Ninja -S runtimes -B build -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind;pstl" -DPSTL_PARALLEL_BACKEND="gcd"
ninja -C build cxx cxxabi unwind
ninja -C build check-cxx

And everything seems to have passed, although some of the pstl tests said they were “unsupported.”

In any case, I have this code running in another project where it is thoroughly tested, so probably there are no serious problems in it. The delay was mostly in adding all of the “__” everywhere and reformatting the code for llvm.