[builtins] Add __builtin_assume_separate_storage.

[davidtgoldblatt]

Summary

Just plumbing from the language level to the assume intrinsics with
separate_storage operand bundles.

[davidtgoldblatt]

Updating per review comments.

[davidtgoldblatt]

Updating per review comments.

[davidtgoldblatt]

Fix out-of-date comment.

[davidtgoldblatt]

Per review comments.

[davidtgoldblatt]

Rebase

[bruno]

Nice, thanks for adding the builtin layer.

clang/lib/Sema/SemaChecking.cpp

7808

Can you prefix this with a FIXME/TODO?

7810

IIUC, seems like there a two pieces here you can already cover as part of this patch:

• Check for exact two arguments. If in the future an optional multi-arg variadic version is to be supported the checking could be enhanced/changed to support that.
• Check both are pointers. I noticed that in the testcase you rely on incompatible integer to pointer conversion and friends to catch some of these, but a more user friendly diagnostic could be explicit about the expectations. In case these current error diagnostics you are relying run before this sema check, perhaps a note diagnostic could help clarify.

[bruno]

Additionally, this likely deserves an entry in ./clang/docs/ReleaseNotes.rst.