[clang] [clang][ExtractAPI] Compute inherited availability information (PR #103040)

[Daniel Grumberg via cfe-commits]

daniel-grumberg wrote:

> > Additionally this computes availability information for all platforms ahead
> 
> Can you add a test for this?

The behavior is not exposed yet so I can't add a test, will do when I start exposing the behavior.

https://github.com/llvm/llvm-project/pull/103040