[Tooling] Add a hack to work around issues with matcher binding in r368681.

The change in r368681 contains a (probably unintentional) behavioral change for
rewrite rules with a single matcher. Previously, the single matcher would not
need to be bound (`joinCaseMatchers` returned it directly), even though a final
DynTypeMatcher was created and bound by `buildMatcher`. With the new change, a
single matcher will be bound, in addition to the final binding (which is now in
`buildMatchers`, but happens roughly at the same point in the overall flow).

This patch simply duplicates the "final matcher" trick: it creates an extra
DynTypedMatcher for each rewrite rule case matcher, and unconditionally makes it
bindable. This is probably not the right long-term fix, but it does allow
existing code to continue to work with this interface.

Subscribers: cfe-commits, gribozavr, ymandel

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66273

llvm-svn: 368958