[flang][OpenMP] Semantic checks for TASKGRAPH (#160115)
This verifies the "structural" restrictions on constructs encountered in
a TASKGRAPH construct.
There are also restrictions that apply to list items, specifically in
the following contexts:
- a list item on a clause on a replayable construct,
- data-sharing attributes for a variable on a replayable construct.
These restrictions are not verified, because that would require knowing
which clauses (on a potential compound directive) apply to the task-
generating construct of interest. This information is not available
during semantic checks.
