_isOptional(type(of: value)) Does Not Do What You Think It Does (#28994)

In particular, if value is `Any` in a generic context, then `type(of: value)` is
`Any.Protocol` which is never considered optional.  As a result, the first
clause here was never actually being used for `print()` or other similar paths.
(Curiously, it _was_ used for string interpolation.)

This changes how we test for an optional type so that the first clause is consistently used for all optionals, even when they are wrapped in `Any` containers.

Fortunately? `print()` was producing the right results for
optionals because of a dynamic cast bug that failed to
unwrap optionals in these same contexts. <sigh>

Author: tbkka

stdlib/public/core/OutputStream.swift

@@ -384,7 +384,9 @@ internal func _print_unlocked<T, TargetStream: TextOutputStream>(
   // string. Check for Optional first, before checking protocol
   // conformance below, because an Optional value is convertible to a
   // protocol if its wrapped type conforms to that protocol.
-  if _isOptional(type(of: value)) {
+  // Note: _isOptional doesn't work here when T == Any, hence we
+  // use a more elaborate formulation:
+  if _openExistential(type(of: value as Any), do: _isOptional) {
     let debugPrintable = value as! CustomDebugStringConvertible
     debugPrintable.debugDescription.write(to: &target)
     return