[Parser] Don't code-complete twice.

When we are consuming the current token just to enter a new token stream, we push
the current token in the back of the stream so that we get it again.

Unfortunately this had the effect where if the current token is a code-completion one,
we would code-complete once during consuming it and another time after the stream ended.

Fix this by making sure that, in this case, ConsumeAnyToken() will consume a code-completion
token without invoking code-completion.

rdar://12842503