This adds a special case for traversing CXXOperatorCallExpr. Its
children are traversed in the order in which they appear in the source
code, so infix and postfix operators are visited after their first
This behavior was previously only in
LexicallyOrderedRecursiveASTVisitor, moving it here could avoid bugs in
the future. It is still tested in
LexicallyOrderedRecursiveASTVisitorTest.cpp in VisitTemplateDecl.
Clients that somehow rely on the original traversal order will have to
be updated though.