[flang] Resolve and check names in equivalence sets

Collect sets of `parser::EquivalenceObject` to process at the end of
the specification part. This is so that names mentioned in the
EQUIVALENCE statement don't trigger implicit declarations.

The `EquivalenceSets` class performs most of the numerous checks
on objects that can be in equivalence sets at all and objects that
can be in them together. It also merges sets when the same object
appears in more than one.

Once equivalence sets are checked they are added to the `Scope`.
Further checks will be necessary after the size and alignment of
variables are computed.

Add `FindUltimateComponent` to simplify checks on ultimate components
of derived types. Use it to implement `HasCoarrayUltimateComponent`
and checks on equivalence objects.

Make `ExpressionAnalyzer::Analyze(Designator)` public so that
`parser::EquivalenceObject` can be analyzed.

Add `GetDefaultKind`, `doublePrecisionKind`, and `quadPrecisionKind`
to `SemanticsContext` so that `defaultKinds_` does not need to be
accessed directly.

Original-commit: flang-compiler/f18@1cc898e5b8d5dbda19748d8bc2ee8cba5c7df4ec
Reviewed-on: https://github.com/flang-compiler/f18/pull/494
Tree-same-pre-rewrite: false