swift234

fix rdar://24029542 "Postfix '.' is reserved" error message" isn't helpful

This adds some heuristics so we can emit a fixit to remove extraneous
whitespace after a . and diagnose the case where a member just hasn't
been written yet better.  This also improves handling of tok::unknown
throughout the parser a bit.

This is a re-commit of ff4ea54 with an update for a SourceKit test.