[stdlib] Remove _Unwrappable

[lorentey]

There are no uses of the stdlib's _Unwrappable protocol, so there is no reason to keep it as ABI; we should remove it.

[lorentey]

This was uncovered by @jrose-apple's improved diagnostics in #20485.

[lorentey]

@swift-ci please smoke test

[jrose-apple]

This was a necessary workaround at one point. What changed?

[lorentey]

/home/buildnode/jenkins/workspace/swift-PR-Linux-smoke-test/branch-master/swift/stdlib/public/core/BridgeObjectiveC.swift:697:38: error: use of undeclared type '_Unwrappable'
00:25:32.196 if !done, let wrapper = source as? _Unwrappable {
00:25:32.196 ^~~~~~~~~~~~
00:25:32.196 ninja: build stopped: subcommand failed.

Ah, I see, grep failed me -- we still need it. We'll just want to rename unwrap to _unwrap, then.

[lorentey]

Closing; I will submit a new PR to rename unwrap in a moment.

[lorentey]

New PR is up; see #20511.