[clang][ExtractAPI] Record availability information only for the target platform (#76823)

Currently, ExtractAPI provides availability information for all
platforms within a given domain. With this change, we narrow down the
output to include availability details only for the specified target
platform, so users can generate the symbol graph with only the
availability information they need, omitting information of the other
platforms.

This change reverts the functionality introduced in
[`57c9780d`](https://github.com/llvm/llvm-project/commit/57c9780).

rdar://120419037