[clang] [clang] Add `__has_feature(swiftcc)` support (PR #85347)

[Arnold Schwaighofer via cfe-commits]

https://github.com/aschwaighofer requested changes to this pull request.

In a past review (https://github.com/llvm/llvm-project/pull/72159), @AaronBallman  (https://github.com/AaronBallman) noted that the existing use of `FEATURE` (as in `FEATURE(swiftasynccc,...`) was not the right qualifier to use. And it should have been an `EXTENSION` instead. We should use `EXTENSION` for the `swiftcc` check.

Quoting from the top of the file:
```
// FEATURE(...) should be used to advertise support for standard language       
// features, whereas EXTENSION(...) should be used for clang extensions. Note   
// that many of the identifiers in this file don't follow this rule for backward
// compatibility reasons.
```

https://github.com/llvm/llvm-project/pull/85347