SI-6406 Restore deprecated API

The original patch for SI-6406 was intended for 2.10 but during
those volatile weeks of early autumn, it missed the boat.

A deprecated method was incorrectly tagged at 2.10 and later
removed; this restores the method and its test, and resets
the deprecation clock to 2.11.

The deprecation tool should confirm that changes occur on the
git timeline as claimed.
Improve code style in the Scaladoc implementation.

It fixes the following inefficiences or code style violations:

	- Explicit asInstanceOf calls.
	- Boxing symbols instead of using plain NoSymbol.
	- Matching `this` instead of late-binding.
make treeInfo more globally visible
SI-6815 untangle isStable and hasVolatileType

`Symbol::isStable` is now independent of `Symbol::hasVolatileType`,
so that we can allow stable identifiers that are volatile in ident patterns.

This split is validated by SI-6815 and the old logic in RefChecks,
which seems to assume this independence, and thus I don't think ever worked:

```
if (member.isStable && !otherTp.isVolatile) {
  if (memberTp.isVolatile)
    overrideError("has a volatile type; cannot override a member with non-volatile type")
```

Introduces `admitsTypeSelection` and `isStableIdentifierPattern` in treeInfo,
and uses them instead of duplicating that logic all over the place.

Since volatility only matters in the context of type application,
`isStableIdentifierPattern` is used to check patterns (resulting in `==` checks)
and imports.
remove unused methods: checkStable, isStableExceptVolatile
Merge pull request #2519 from som-snytt/issue/regex-revert

SI-6406 Restore deprecated API
Merge pull request #2374 from adriaanm/ticket-6815

SI-6815 untangle isStable and hasVolatileType
Remove self types check suppression usage from scaladoc.

Also remove private setting not used anymore.
No longer cache all subclass templates.

Instead only cache direct subclasses and compute all known subclasses
as a transitive closure.
Merge pull request #2454 from vigdorchik/scaladoc_fixes

This is a cleanup of scaladoc code.