Defer analysis of large switch bodies

This presents a regression in diagnostic quality that is definitely
worth it not to lie to SILGen about whether a switch is covered or not.

At the same time, disable SIL’s unreachable diagnostic for ‘default’
clauses which would previously cause a warning to be emitted if the
default was proven to be unreachable.  This analysis is incomplete
anyways and can be done by Sema in the future if we desire.

lib/SILGen/SILGenPattern.cpp

@@ -1031,7 +1031,8 @@ void PatternMatchEmission::emitDispatch(ClauseMatrix &clauses, ArgArray args,
         } else {
           Loc = clauses[firstRow].getCasePattern()->getStartLoc();
         }
-        SGF.SGM.diagnose(Loc, diag::unreachable_case, isDefault);
+        if (!isDefault)
+          SGF.SGM.diagnose(Loc, diag::unreachable_case);
       }
     }
   }