[APINotes] Prefer diff -u over diff -b
Lit's internal shell does not support -b (properly). This patch rewrites
the tests to conform to their existing semantics (using
--implicit-check-not) using diff -u. This allows for enabling lit's
internal shell by default on all platforms.
Fixes #106249.
Reviewers: compnerd, ilovepi, petrhosek, DougGregor
Reviewed By: ilovepi, compnerd
Pull Request: https://github.com/llvm/llvm-project/pull/157588
