P2642: Introduce least_multiple_at_least(x,y)

This is a math-font wording macro.
It replaces the repeated phrase
"the least multiple of x greater than or equal to y."
This change addresses two issues.

1. LWG requested that we replace the repeated phrase
   with a "specification macro."  We use math font
   rather than the usual all-capital-letters
   because the "macro" operates on mathematical quantities,
   not code.  (Contrast with GENERALIZED_SUM or DECAY.)

2. It attempts (at least partly) to address Issue
   ORNL#442
   by defining the meaning of this phrase when x is zero.