[TypeChecker] [NFC] Make unreachable case unreachable

A part of swift::diagnoseExplicitUnavailability() that should never run for deprecations looks for them and handles them with “break”. Change this to llvm_unreachable() so we notice if this somehow happens.

This change *should* be NFC; I’m splitting it out from other changes so that bisections can point to it if it causes problems in the future.

Author: beccadax

lib/Sema/TypeCheckAvailability.cpp

@@ -2116,7 +2116,7 @@ bool swift::diagnoseExplicitUnavailability(
   auto &diags = ctx.Diags;
   switch (Attr->getPlatformAgnosticAvailability()) {
   case PlatformAgnosticAvailabilityKind::Deprecated:
-    break;
+    llvm_unreachable("shouldn't see deprecations in explicit unavailability");
 
   case PlatformAgnosticAvailabilityKind::None:
   case PlatformAgnosticAvailabilityKind::Unavailable: