Use descriptive enums instead of Boolean parameters

[tautschnig]

The semantics of true or false are often hard to understand without extensive context. Using enums (with just two values, but descriptive identifiers) may be a better route.

[hannes-steffenhagen-diffblue]

As an example for this, simplify_expr returns true when the expression was simplified and false when it wasn't. If you see code like if(simplify_expr(expr)) you need to know this to read the code; If it was something like if(simplify_expr(expr) == Simplify::ChangedExpression) it'd be easier to discern the meaning (this isn't a parameter in this case, but basically the same logic applies).

It's certainly worth having a discussion about the places where we use "meaningful" booleans like this (i.e. where a boolean is used as a generic two-value thing instead of an actual True/False thing).