[flang] [OpenMP] parse tree changes for ATOMIC constructs (flang-compiler/f18#636)

1. make the parse tree nodes more conform with OpenMP spec

2. isolate the memory related clauses to make the parse tree nodes
   extendable for OpenMP 5.0

3. source provenance is saved for each atomic-clause (read, write, update,
   and capture); for atomic-clause that is not present, source location
   is saved for "ATOMIC" directive name itself

More tests will be added during Semantics.

Original-commit: flang-compiler/f18@8e2db2f868f9dce705aa0a504e1c9ac1af8fe08f
Reviewed-on: https://github.com/flang-compiler/f18/pull/636