Begin RAV reunification: s/DataRecursiveASTVisitor/RecursiveASTVisitor/

Define a macro to reduce the delta between RecursiveASTVisitor and
DataRecursiveASTVisitor.

Some years ago, someone thought it was a good idea to copy the entire RAV to
support cursor visitation in libclang.

Since then the two have diverged needlessly and the rest is history.

This series of commits aims to reduce delta between the two so that we can
reason about their differences, apply bugfixes that were only made to one but
not the other, and ultimately find a way to unify two two chunks of code.

llvm-svn: 209091