swift234

Sema: Move potential unavailability diagnostics into MiscDiagnostics.

This commit moves potential unavailability diagnostics (when not treating unavailable
symbols as optional) out of CSApply.cpp and into MiscDiagnostics.cpp, alongside the
deprecation-based and explicitly unavailable checks. This move is a precursor to adding
support for separate availability information on getters and setters. No intended
functional change.

Swift SVN r24836