[clang] [Clang] Implement the 'counted_by' attribute (PR #76348)

[Sean McBride via cfe-commits]

seanm wrote:

@bwendling thanks for your reply. No idea how representative it is of other projects, but 3 of the 6 flexible arrays in [libusb](https://github.com/libusb/libusb) structures have these kinds of non-trivial counts.

The GCC folks seem to have [considered](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896) adding such support, but it's not clear what, if anything, they decided.

https://github.com/llvm/llvm-project/pull/76348