Patch: bug fix for PR18218

[Matt Calabrese]

Ah, I'm out of my domain, I didn't realize the wording of [c.math] 11
in addition to [global.functions] was what was causing the need for
the templates... I think I'm starting to understand the full issues
here. One solution might be something along these lines:

1) Declare the overloads that are explicitly specified. These are just
3 overloads total for each IIUC.
2) Declare a template overload that takes r-value references for each
argument that is enabled if and only if the corresponding function
would be callable with the forwarded arguments (this can be done with
a metafunction implemented via SFINAE). This function would internally
cast the arguments to the corresponding type and invoke the actual
function (therefore it exhibits the same observable behavior as if
this template didn't exist).
3) Declare a second template overload that takes r-value references
for each argument but that is DISABLED if the corresponding function
would be callable with the forwarded arguments. This overload would be
the one that does something along the lines of the trick that was
originally implemented (though adjusted for r-value refs). It would do
an is_arithmetic check rather than the __numeric_type trick.

The overall result would be that anything that would have been
callable with only the original overloads would still work and have
the same behavior. The enable_if makes sure that the fallback would be
used only in the cases where such a call would have failed with only
the explicitly specified overloads. I think this satisfies all of the
standard's requirements.