[dbus-c++] DBus-C++ operator overloading should rely on ADL

DBus-C++ operator overloading (operator >>/<<) should rely on
argument-dependent lookup, rather than on sticking the overloads in the
global namespace, so that they can always be found correctly when used in
templates.

See http://clang.llvm.org/compatibility.html#dep_lookup to understand why the
existing behaviour was incorrect

BUG=none
TEST=Compile against types.h with Clang

Change-Id: I9239e960f6872f0f312561050d1bbd4cc9b87458
Reviewed-on: https://gerrit.chromium.org/gerrit/42027
Tested-by: Liam McLoughlin <lmcloughlin@chromium.org>
Commit-Queue: Ryan Sleevi <rsleevi@chromium.org>
Reviewed-by: Ryan Sleevi <rsleevi@chromium.org>
4 files changed