[+0-all-args] Knock out some inappropriate "unreachable"s in the runtime.

Future Swifts may add new metadata kinds, so it isn't appropriate to crash when
we see one. In the case of reflection, we can fall back to opaque behavior.

rdar://34222540

Author: jckarter

stdlib/public/runtime/ReflectionMirror.mm

@@ -635,19 +635,20 @@ auto call(OpaqueValue *passedValue, const Metadata *T, const Metadata *passedTyp
 
     /// TODO: Implement specialized mirror witnesses for all kinds.
     case MetadataKind::Function:
-    case MetadataKind::Existential: {
-      OpaqueImpl impl;
-      return call(&impl);
-    }
+    case MetadataKind::Existential:
+      break;
 
     // Types can't have these kinds.
     case MetadataKind::HeapLocalVariable:
     case MetadataKind::HeapGenericLocalVariable:
     case MetadataKind::ErrorObject:
       swift::crash("Swift mirror lookup failure");
-  }
+    }
 
-  swift_runtime_unreachable("Unhandled MetadataKind in switch.");
+    // If we have an unknown kind of type, or a type without special handling,
+    // treat it as opaque.
+    OpaqueImpl impl;
+    return call(&impl);
 }
 
 } // end anonymous namespace
@@ -738,7 +739,7 @@ char swift_reflectionMirror_displayStyle(OpaqueValue *value, const Metadata *T)
       return "(ErrorType Object)";
   }
 
-  swift_runtime_unreachable("Unhandled MetadataKind in switch.");
+  return "(Unknown)";
 }
 
 #if SWIFT_OBJC_INTEROP