[C++11] DR1479 - Literal operators and default arguments

[Ed Smith-Rowland]

I tried this patch a year ago and, as pointed out by NightStrike, got 
lost in the shuffle.

The idea is that default arguments in literal operators would lead to 
ambiguities and might not act like the uthor intends.

The earlier patch errors out.  This time I just warn.  I'm not sure what 
we want here actually so some feedback would be nice.
I could alternatively error and add a note about default argument.

Builds and tests clean on x86_64-linux.

OK?

Ed