[AST] Traverse CXXOperatorCallExpr in LexicallyOrderedRecursiveASTVisitor

Summary:
This affects overloaded operators, which are represented by a
CXXOperatorCallExpr whose first child is always a DeclRefExpr referring to the
operator. For infix, postfix and call operators we want the first argument
to be traversed before the operator.

Reviewers: arphaman

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D37200

llvm-svn: 312633