swift234

[cmake][utils] Add a new utility function precondition_binary_op.

This is a flexible way to assert that a binary operation succeeds. For
instance to precondition equality:

  precondition(EQUAL FOO BAR)

or string equality:

  precondition(STREQUALS FOO BAR)

Since cmake is so loosely typed, it is really important that we
precondition as much as possible to ensure sane behavior.