[libc++][NFC] Define functor's call operator inline

This fixes a mismatched visibility attribute on the call operator in
addition to making the code clearer. Given this is a simple lambda
in essence, the intent has always been to give it inline visibility.

GitOrigin-RevId: 21a1a263a6d9c0c44ef8eb0744786e2aa5d59e53
1 file changed