[clang] [BoundsSafety] Allow 'counted_by' attribute on pointers in structs in C (PR #90786)

[Dan Liew via cfe-commits]

================
@@ -335,6 +336,22 @@ Attribute Changes in Clang
 - Clang now warns that the ``exclude_from_explicit_instantiation`` attribute
   is ignored when applied to a local class or a member thereof.
 
+- The ``counted_by`` attribute can now be late parsed in C when
----------------
delcypher wrote:

So to add some context here. The original reason there was a release note is because in #88596 (which this patch builds upon) @AaronBallman asked for there to be a release note about the new `-fexperimental-late-parse-attributes` flag. This patch means `-fexperimental-late-parse-attributes` actually does something now (before it did nothing) so I updated the release notes on that flag and it "felt" like it made sense to explain in the release notes the change on the attributes in relation to the flag.

I don't have strong opinions here. I'm ok to drop the changes to the release notes or keep them or something in between. @AaronBallman any opinions here?

https://github.com/llvm/llvm-project/pull/90786