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

[via cfe-commits]

mydeveloperday wrote:

> The only problem I see is how clang-format parses those lists from yaml. I think there is no possibility to choose between adding or overwriting, if you put `AttributeMacros` in your `.clang-format`, but it just overwrites the values.

if they can be overridden!, then I'm ok with putting them either in the Google style or the default style.

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