[flang] [OpenMP] miscellaneous parse tree updates (flang-compiler/f18#673)

1. Big chunk: update comments in parse-tree.h and openmp-grammar.h
   with Spec chapter/section info, simple explanation, or productions.

2. Update `To`, `Link`, and `From` clauses with `OmpObjectList` to allow
   `/Common Block/`. Spec does not mention whether `Common Block name`
   should be accepted or not, so we should assume that these clauses
   accept normal `list-item`, which is `Variable`, `Array Section`, or
   `Common Block name`.

Original-commit: flang-compiler/f18@140315cb62da295f6306c2afc737041fa1626dff
Reviewed-on: https://github.com/flang-compiler/f18/pull/673