ASTMatchers: Keep AllCallbacks in a set instead of a vector

AllCallbacks is currently only used to call onStartOfTranslationUnit and
onEndOfTranslationUnit on them. In this (and any other scenario I can
come up with), it is important (or at least better) not to have
duplicates in this container. E.g. currently onEndOfTranslationUnit is
called repeatedly on the same callback for every matcher that is
registered with it.

llvm-svn: 249598