[flang] Structural checks for DO construct (flang-compiler/f18#536)

Most restrictions in 2.7.1 Loop construct, nesting checks are still on TODO list.

seenClauses (EnumSet) is dropped. A multimap clauseInfo is added to save the encountering clause and its parser tree node pointer (parser::OmpClause) and to do the checks after walking through all the clauses.

Original-commit: flang-compiler/f18@f79735768265ace3dcd2a5e1f7fbe4d770c379f6
Reviewed-on: https://github.com/flang-compiler/f18/pull/536