[clang] [clang-format] Add support for absl nullability macros (PR #130346)

[via cfe-commits]

mydeveloperday wrote:

Imagine if we did this for every "toolkit"... we'd be inundated with add this/that etc... why can't people who use Abseil not add them to their .clang-format? Outside of google are that many people even using Abseil?

Abseil itself doesn't even define them  in their .clang-format file.

This is a no from me.

https://github.com/llvm/llvm-project/pull/130346