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

[Owen Pan via cfe-commits]

owenca wrote:

> > I wish they had not been added to the default but can't take them off the lists for backward compatibility. I don't think we should add more. The reason is that people wouldn't be able to turn off the special meaning if they wanted to use e.g. `absl_nonnull` as a regular identifier.
> 
> What if this is limited to the Google style? +cc some folks involved in the Google style @ilya-biryukov It is not yet in the style guide, but we are working on getting the style guide updated.

I would have no problem with that.

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