[flang] OpenMP structural check framework (PARALLEL as example) (flang-compiler/f18...

[flang] OpenMP structural check framework (PARALLEL as example) (flang-compiler/f18#493)

This patch is to show the framework to do OpenMP structure related check and you may find missing/incomplete implementation, which will be added in the future.

1. Each encountering construct has its own context and the context info will be filled while walking the directive/clause nodes. In the `Leave`, the current OpenMP context will be popped out.

2. When entering the construct, necessary nesting check will be performed. Use std::vector to implement the context stack because certain nesting restrictions may need to trace back more than one level.

3. PARALLEL construct is used as an example for clause validity check.

4. `EnumSet` is used to represent the directive/clause sets and to show the error msg. All the error msgs are quoted from the specification.

5. Necessary `CharBlock` is added for error msgs.

Original-commit: flang-compiler/f18@620441c03e21ae28aed3864b9581cd6cc1705c65
Reviewed-on: https://github.com/flang-compiler/f18/pull/493