clang: fix typo availbility

- Fixes [[ https://github.com/llvm/llvm-project/issues/56787 | #56787 ]].

I am fixing the spelling of availability.

I am unsure if this change will have any side effects. If someone can
help on how to check if it has any side effects, I can test those out as
well.

Reviewed By: inclyc

Differential Revision: https://reviews.llvm.org/D131277