swift234

[Serialization] Don't serialize non-public documentation

Let's say I am a good citizen and document my private symbols:

    /** My TOP SECRET DOCUMENTATION */
    private class Foo {
    }

When I go to distribute the compiled binary, I find out my private
documentation is distributed as well:

    $ swiftc test.swift -emit-module -module-name "test"
    $ strings test.swiftdoc
    My TOP SECRET DOCUMENTATION
    /** My TOP SECRET DOCUMENTATION */

If a client can't use a symbol (e.g. it's private [or internal and not
-enable-testing]) don't emit the documentation for a symbol in the
swiftdoc.

Fixes: SR-762, rdar://21453624

The test coverage implements this truth table:

| visibility | -enable-testing | documentation? |
|------------|-----------------|----------------|
| private    | no              | ❌             |
| internal   | no              | ❌             |
| public     | no              | ✅             |
| private    | yes             | ❌             |
| internal   | yes             | ✅             |
| public     | yes             | ✅             |

Modified the existing comments test coverage to expect non-public
documentation not to be emitted.

Don't rely on existing comment structure

Refuse to emit comments if the decl cannot actually have one.  To
accomplish this, we move `canHaveComment` into the Decl instance.  It
must also be marked `const`, since one of its existing usages operates
on a const pointer.

Perform fewer checks when serializing the standard library.