swift234

[QoI] make several improvements to the unused expression diagnostics, to go
along with recent policy changes:

- For expression types that are not specifically handled, make sure to
  produce a general "unused value" warning, catching a bunch of unused
  values in the testsuite.

- For unused operator results, diagnose them as uses of the operator
  instead of "calls".

- For calls, mutter the type of the result for greater specificity.

- For initializers, mutter the type of the initialized value.

- Look through OpenExistentialExpr's so we can handle protocol member
  references propertly.

- Look through several other expressions so we handle @discardableResult
  better.