swift234

Use a unique constraint locator path for the operand of a
checked cast expression.

We don't actually *use* that path for anything right now,
because we basically re-check the cast from scratch after
constraint application.  This is nonetheless necessary to
avoid collisions with constraints which might be located
on the result, such as would arise in an initialization
context.

In particular, this patch fixes a crash arising when both
the operand and the result of a coercion require a
user-defined conversion.  Test to follow.

Swift SVN r18386