Parse: Don't parse after the eof has been consumed

ParseCXXNonStaticMemberInitializer stashes away all the tokens for the
initializer and an additional EOF token to denote where the initializer
ends.  However, it is possible for ParseLexedMemberInitializer to get
its hands on the "real" EOF token; since the two tokens are
indistinguishable, we end up consuming the EOF and descend into madness.

Instead, make it possible to tell which EOF token we are looking at.

This fixes PR21872.