[flang] [OpenMP] Canonicalization framework (flang-compiler/f18#599)

* [OpenMP] Canonicalization framework

This is mainly designed for loop association work but can be used for others,
and `CanonicalizeOmp` must be after `CanonicalizeDo`.

At the `Block` level, recognize legal sequence of `OpenMPLoopConstruct`,
`DoConstruct`, and `OmpEndLoopDirective`. Move available `DoConstruct`
and optional `OmpEndLoopDirective` into `OpenMPLoopConstruct`. Throw error
messages if:
 1. `DoConstruct` is not following `OpenMPLoopConstruct`
 2. `OmpEndLoopDirective` is not following associated do-loop

Once this pass this done, Semantics will not proceed if error exists.

* Update on reviews

1. extract matching and move part into its own function (once `DoConstruct`
   is moved, see whether `OpenMPEndLoopDirective` is available)

2. Use a template function to access construct from ExecutionPartConstruct.

3. Move this code into namespace semantics

Original-commit: flang-compiler/f18@52979f1e93f40c31e04118067403a258555d139a
Reviewed-on: https://github.com/flang-compiler/f18/pull/599