[v3] support for disabling concept checking

[Benjamin Kosnik]

yeah I was tempted to just turn them off by default with this patch. 
Perhaps I should have done so.

>    (no debug info) : 1172252
>    -g  : 5984348
>    -g -D_STL_NO_CONCEPT_CHECKS : 4112244
>    -g -gdwarf-2 : 6039964
>    -g -gdwarf-2 -D_STL_NO_CONCEPT_CHECKS : 4023208
>    -g -gdwarf-2 -feliminate-dwarf2-dups :  7391764
>    -g -gdwarf-2 -feliminate-dwarf2-dups -D_STL_NO_CONCEPT_CHECKS : 5271560

yeah. Perhaps I should still do that. Thoughts?

-benjamin