Fix equalsNode() to accept pointers to derived types.

Summary:
Move the 'const' in the AST_*MATCHER_P* macros to the right of ParamType to
avoiad applying the constness on the wrong level when ParamType is a pointer.
Change equalsNode() to explicitly accept 'const Decl*' or 'const Stmt*'.

Reviewers: klimek

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D3994