Previously we would wait until CSApply, which
would trigger their type-checking in
`coercePatternToType`. This caused a number of
bugs, and hampered solver-based completion, which
does not run CSApply. Instead, form a conjunction
of all the ExprPatterns present, which preserves
some of the previous isolation behavior (though
does not provide complete isolation).
We can then modify `coercePatternToType` to accept
a closure, which allows the solver to take over
rewriting the ExprPatterns it has already solved.
This then sets the stage for the complete removal
of `coercePatternToType`, and doing all pattern
type-checking in the solver.
Currently ExprPatterns are type-checked during
CSApply. As such, they can cause solution
application to fail with a pattern that isn't
well-formed. I'm planning on moving their
type-checking into the solver, but until then,
lets only do limited exhaustiveness checking for
the switch if there was an error.
rdar://105781521
