[libc++] Add stdlib header: ratio

Adjusted copy of the libc++ implementation in LLVM 11.0:
https://github.com/llvm/llvm-project/blob/release/11.x/libcxx/include/ratio
4 jobs for libcxx-add-ratio
Status Name Job ID Coverage
  Git Update
canceled git-update #432878
nix

 
  Tests
canceled tests-on-llvm-10 #432880
allowed to fail manual

canceled tests-on-llvm-11 #432881
nix

canceled tests-on-llvm-9 #432879
nix