libdiff: More type issues.

Sponsored by: Klara, Inc.
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D45080

libdiff: Improve function prototype detection.

- Recognize ObjC methods.
- Start searching within the leading context.

Sponsored by: Klara, Inc.
Reviewed by: thj
Differential Revision: https://reviews.freebsd.org/D44301

Add 'contrib/libdiff/' from commit '9eb461aa4b61ab47855b2cee9e5b626a76888b5e'

git-subtree-dir: contrib/libdiff
git-subtree-mainline: f6d489f402c320f1a6eaa473491a0b8c3878113e
git-subtree-split: 9eb461aa4b61ab47855b2cee9e5b626a76888b5e

Reviewed by: imp
Sponsored by: Klara, Inc.