Skip some unnecessary type checks.

Summary:
Skip some unnecessary type checks wrt DynTypedNodes.
Add DynTypedNode::getUnchecked() to skip the runtime check when the type
is known.
Speed up DynTypedNode::operator== by using isSame() instead of
isBaseOf().
Skip the type check in MatcherInterface<T>::matches(). All calls come
from DynTypedMatcher::matches(), which already did the type check.
This change speeds up our clang-tidy benchmark by ~4%.

Reviewers: klimek

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D6468