[Swiftify] Adjust _SwiftifyImport invocation to align with the signature

of the macro in the currently laoded macro module

Stdlib macros are shipped with the SDK, so we need to make sure that the
way we invoke the macro matches the version of the macro in the SDK.
This patch skips the `spanAvailability` parameter if it isn't present.
There's no good way to test this, because we will always pick up the
current macro in our llvm-lit test suite, but I've tested it manually.

rdar://152278292

stdlib/public/core/SwiftifyImport.swift

@@ -104,4 +104,4 @@ public func _swiftifyOverrideLifetime<
   // TODO: Remove @_unsafeNonescapableResult. Instead, the unsafe dependence
   // should be expressed by a builtin that is hidden within the function body.
   dependent
-}
+}