Support `__swift_attr__` to APINotes

[kateinoigakukun]

Motivation

Currently, we have no way to mark Clang-imported declarations with arbitrary Swift attributes with APINotes files. For example, it can be problematic when migrating to strict concurrency model because we can't add concurrency related markers without modifying system header files.

Proposed solution

Add SwiftAttr field to APINotes like below

Name: SwiftWASILibc
Globals:
- Name: errno
  SwiftAttr: ["nonisolated(unsafe)"]

Alternatives considered

No response

Additional information

No response