Don't allow assignment in an assert

[Chuck.Esterbrook]

Asserts should have no side effects as they may be excluded (-include-asserts:no or -turbo). Particularly, the top level expression should not be an assignment as this is usually just a typo where the programmer meant to say "x == y", not "x = y". This often gives a warning:

warning: The expression "..." (of type "...") will always evaluate to true because it is not nilable. You can remove the expression.

It should always give an error with a more appropriate message.

[hopscc]

I'm providing patch to fix this.

Assignments as an assert condition generate an error
Conditions that generate a 'always true cos not nilable' warning have
the warning message augmented with corrections specific to an assert ( the actual wording of this could probably be improved)

[Chuck]

I've made a further enhancement to this that if the condition .willChangeVar then an error will be generated. In addition to a top-level assignment expression, this also covers nested expressions, 'out' parameters and all forms of augmented assignment. See changeset:1545