sprinkle llvm_unreachable for covered switches (NFC)

Annotate the covered switches with `llvm_unreachable` to avoid the MSVC
warning which does not recognise the covered switches.  This allows us
to avoid a spew of warnings.

lib/Sema/ConstraintSystem.cpp

@@ -4077,6 +4077,7 @@ ConstraintSystem::isConversionEphemeral(ConversionRestrictionKind conversion,
     // parameter.
     return ConversionEphemeralness::NonEphemeral;
   }
+  llvm_unreachable("invalid conversion restriction kind");
 }
 
 Expr *ConstraintSystem::buildAutoClosureExpr(Expr *expr,
@@ -4381,6 +4382,7 @@ bool SolutionApplicationTarget::contextualTypeIsOnlyAHint() const {
   case CTP_CannotFail:
     return false;
   }
+  llvm_unreachable("invalid contextual type");
 }
 
 /// Given a specific expression and the remnants of the failed constraint