[flang] Add FORALL checking to DoChecker

FORALL statements and constructs require a lot of the same checking
as DO CONCURRENT, so do the checks in DoChecker so that code can be
shared where possible. This requires some reorganization there.
Remove code from AssignmentChecker that did some of these checks.

Change names that contain `DoVar` or `DoVariable` to `IndexVar` to
reflect the fact that they may be DO or FORALL index variables.
Distinguish between the two when necessary with enum `IndexVarKind`.

Change some messages that referred to "concurrent-header" or
"concurrent-control" to specifically say "DO CONCURRENT" or "FORALL".

Original-commit: flang-compiler/f18@84752c492e910573e2f0ede1ed3c0417aac363b9
Reviewed-on: https://github.com/flang-compiler/f18/pull/989
Tree-same-pre-rewrite: false