UML14 — Add "Let"-like construct to OCL

Summary: Currently if a sub-expression within an OCL expression is
used twice or more, you need to spell it out each time. This
is cumbersone, eror-prone and it also disguises to the reader
of the constraint that these sub-expressions are in fact identical.

The proposal is to add a construct to OCL, that allows one
to define a variable which holds the value of such a sub-expression.

Redundant with issue 1788.