Filter the toplevel matchers by kind.

Summary:
Filter the toplevel matchers by kind.
Decl and Stmt matchers are tied to a specific node kind and trying to
match incompatible nodes is a waste.
Precalculate a filtered list of matchers that have a chance of matching
the node and ignore the rest.
Speeds up our clang-tidy benchmark by ~10%

Reviewers: klimek

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D6361