[flang] Improve error messages about component types in allocate checks

Use component visitor framework to attach more information reagrding
where is the ultimate/potential component that is forbidden.
Also remove unused functions.

Original-commit: flang-compiler/f18@4619c5860a43b86f149698c495769bdff2d63557
Reviewed-on: https://github.com/flang-compiler/f18/pull/607
Tree-same-pre-rewrite: false