Move expression pattern solving into the constraint system #63374
Description
As discovered by the stress tester in #63022, we currently rely on CSApply calling coercePatternToType in order to type-check an expression pattern of a switch that's being solved in e.g a conjunction. This hampers solver-based code completion, preventing us from offering completions in expression pattern position. We ought to solve the expression as part of the constraint system.
rdar://104954155