rustc: Don't ICE if we invalidate an invalid incr dir

This showed up on the Windows bot for testing this PR, and this pr allows
`mark_incr_comp_session_as_invalid` ok if it's already invalid, hopefully
avoiding scary ICEs and instead leaving the nicely printed errors

Author: alexcrichton

src/librustc/session/mod.rs

@@ -642,6 +642,7 @@ impl Session {
             IncrCompSession::Active { ref session_directory, .. } => {
                 session_directory.clone()
             }
+            IncrCompSession::InvalidBecauseOfErrors { .. } => return,
             _ => bug!("Trying to invalidate IncrCompSession `{:?}`",
                       *incr_comp_session),
         };