Let RecursiveASTVisitor visit array index VarDecls

An implicit copy ctor creates loop VarDecls that hang off CXXCtorInitializer.
RecursiveASTVisitor used to not visit them, so that they didn't show up in the
parent map used by ASTMatchers, causing asserts() when the implicit
DeclRefExpr() in a CXXCtorInitializer referred to one of these VarDecls.

Fixes PR26227.
http://reviews.llvm.org/D16413

llvm-svn: 258503