Special case 0 and 1 matcher in makeAllOfComposite().

Summary:
Remove unnecessary wrapping for the 0 and 1 matcher cases of
makeAllOfComposite(). We don't need a variadic wrapper for those cases.
Refactor TrueMatcher to take advandage of the new conversions between
DynTypedMatcher and Matcher<T>. Also, make it a singleton.
This change improves our clang-tidy related benchmarks by ~12%.

Reviewers: klimek

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D5675