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

[Arnold Schwaighofer via cfe-commits]

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@@ -102,6 +102,9 @@ FEATURE(memory_sanitizer,
 FEATURE(thread_sanitizer, LangOpts.Sanitize.has(SanitizerKind::Thread))
 FEATURE(dataflow_sanitizer, LangOpts.Sanitize.has(SanitizerKind::DataFlow))
 FEATURE(scudo, LangOpts.Sanitize.hasOneOf(SanitizerKind::Scudo))
+FEATURE(swiftcc,
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aschwaighofer wrote:

I think we should use `EXTENSION` instead of `FEATURE`.

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