Add stubs for sequenced collections.

[cpovirk]

These stubs work around some problems we're seeing, and of course they
may let us identify actual problems in user code. They appear to cause
no problems in environments where the sequenced collections aren't yet
present (e.g., under JDK 11).

See
jspecify/jspecify-reference-checker#192 (comment)
and elsewhere in that thread.

The files here are annotated copies of files from
https://github.com/openjdk/jdk/tree/476d2ae69d6f67fdf9e2a9353f224141318690f2.

Fixes jspecify/jspecify-reference-checker#192.

[wmdietl]

The added annotations all look good and I'm fine with merging.
However, I wonder whether we shouldn't simply pull in a JDK 21, instead of just adding these three files. If we pull in JDK 21 at a later point, there will be conflicts with these files and it is harder to see what the actual changes are.

[cpovirk]

I do expect to pull in JDK21 in the not too distant future. Given my track record recently, though, I'd rather rather not block on it :) (And it will already lead to all sorts of merge conflicts until I sort that process out.) Another option is to either leave the common.collect nullness tests broken (which is ok when we run them only periodically) or to add suppressions, but it's enough suppressions that I'd rather not.

We'll see if that comes back to bite me :)