Revert r156141 (making RecursiveASTVisitor data recursive). It is causing...

Revert r156141 (making RecursiveASTVisitor data recursive).  It is causing clang to blow up in memory usage on -O2 when compiling itself,
which is leading to swapping in some cases when it didn't before.  We need to see if we can make this change
without leading to a massive compile-time bloat.

llvm-svn: 156229