[embedded] Use __has_extension(swiftcc) to detect Swift calling convention

This change replaces the use of `__has_feature(swiftasynccc)` with
`__has_extension(swiftcc)` to detect the SwiftCC availability. The former
condition works fine for most platforms that support both SwiftCC and
SwiftAsyncCC or neither, but it fails for WebAssembly, which supports
SwiftCC but not SwiftAsyncCC.

We add `swiftcc` extension to Clang, and use it here.

Author: kateinoigakukun

stdlib/public/SwiftShims/swift/shims/EmbeddedShims.h

@@ -28,8 +28,9 @@
 extern "C" {
 #endif
 
-// FIXME: Replace with __has_feature(swiftcc) once that's added to Clang.
-#if __has_feature(swiftasynccc)
+// TODO: __has_feature(swiftasynccc) is just for older clang. Remove this
+// when we no longer support older clang.
+#if __has_extension(swiftcc) || __has_feature(swiftasynccc)
 #define SWIFT_CC_swift __attribute__((swiftcall))
 #define SWIFT_CONTEXT __attribute__((swift_context))
 #define SWIFT_ERROR_RESULT __attribute__((swift_error_result))