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

[dlj]

Summary

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.

cfe/trunk/lib/Tooling/Refactoring/Transformer.cpp

 // `TagBase` and the id paired with the case.
 static std::vector<DynTypedMatcher> taggedMatchers(
     StringRef TagBase,
     const SmallVectorImpl<std::pair<size_t, RewriteRule::Case>> &Cases) {
   std::vector<DynTypedMatcher> Matchers;
   Matchers.reserve(Cases.size());
   for (const auto &Case : Cases) {
     std::string Tag = (TagBase + Twine(Case.first)).str();
-    auto M = Case.second.Matcher.tryBind(Tag);
-    assert(M && "RewriteRule matchers should be bindable.");
+    // HACK: Many matchers are not bindable, so ensure that tryBind will work.
+    DynTypedMatcher BoundMatcher(Case.second.Matcher);
+    BoundMatcher.setAllowBind(true);
+    auto M = BoundMatcher.tryBind(Tag);
     Matchers.push_back(*std::move(M));
   }
   return Matchers;
 }
 
 // Simply gathers the contents of the various rules into a single rule. The
 // actual work to combine these into an ordered choice is deferred to matcher
 // registration.