Previously -Winteger-overflow did not do any checking of expressions
whose top-level syntactic form wasn't one of a small list of special
cases. This meant that the warning would appear and disappear depending
on enclosing syntax.
Additionally, if constant evaluation hit a case that it didn't
understand, it would often bail out without visiting subexpressions, so
we wouldn't see warnings on overflow in subexpressions depending on the
form of intervening expressions.
We now walk all evaluated subexpressions of a particular expression when
checking for overflow, and evaluate subtrees rooted on each operation
that might overflow. If such evaluation hits an unevaluatable
subexpression, we switch back to walking the AST looking for
subexpressions to evaluate.
The extra evaluation here also exposed an issue where casts between
complex and fixed-point types would create bogus AST nodes using
CK_IntegralCast, which would lead to the constant evaluator asserting.
That's fixed here too.