Tailored error reporting when dynamic casts have nonsensical type args

This inserts a suitably named function into the stack trace whenever
a dynamic cast failure involves a NULL source or target type.
Very often, crash logs include backtraces with function names but
no log output; with this change, such a backtrace might look like
the following -- note `TARGET_TYPE_NULL` in the function name
here to mark the missing type information:

```
 frame #0: __pthread_kill + 8
 frame swiftlang#1: pthread_kill + 288
 frame swiftlang#2: abort + 128
 frame swiftlang#3: swift::fatalErrorv()
 frame swiftlang#4: swift::fatalError()
 frame swiftlang#5: swift_dynamicCastFailure_TARGET_TYPE_NULL()
 frame swiftlang#6: swift::swift_dynamicCastFailure()
 frame swiftlang#7: ::swift_dynamicCast()
```

Resolves rdar://130630157

stdlib/public/runtime/Casting.cpp

@@ -416,9 +416,48 @@ swift::swift_dynamicCastFailure(const void *sourceType, const char *sourceName,
                     message ? message : "");
 }
 
-SWIFT_NORETURN void swift::swift_dynamicCastFailure(const Metadata *sourceType,
-                                                    const Metadata *targetType,
-                                                    const char *message) {
+SWIFT_NORETURN SWIFT_NOINLINE void
+swift_dynamicCastFailure_SOURCE_AND_TARGET_TYPE_NULL(const char *message) {
+  swift::fatalError(0, "Unconditional cast failed. "
+		    "Both source and target types were NULL. "
+		    "%s\n",
+		    message ? message : "");
+}
+
+SWIFT_NORETURN SWIFT_NOINLINE void
+swift_dynamicCastFailure_SOURCE_TYPE_NULL(const Metadata *targetType, const char *message) {
+  std::string targetName = nameForMetadata(targetType);
+  swift::fatalError(0, "Unconditional cast failed. "
+		    "Source type was NULL, target was '%s' (%p). "
+		    "%s\n",
+		    targetName.c_str(), targetType,
+		    message ? message : "");
+}
+
+SWIFT_NORETURN SWIFT_NOINLINE void
+swift_dynamicCastFailure_TARGET_TYPE_NULL(const Metadata *sourceType, const char *message) {
+  std::string sourceName = nameForMetadata(sourceType);
+  swift::fatalError(0, "Unconditional cast failed. "
+		    "Source type was '%s' (%p), target type was NULL. "
+		    "%s\n",
+		    sourceName.c_str(), sourceType,
+		    message ? message : "");
+}
+
+SWIFT_NORETURN SWIFT_NOINLINE void
+swift::swift_dynamicCastFailure(const Metadata *sourceType,
+				const Metadata *targetType,
+				const char *message) {
+  if (sourceType == nullptr) {
+    if (targetType == nullptr) {
+      swift_dynamicCastFailure_SOURCE_AND_TARGET_TYPE_NULL(message);
+    } else {
+      swift_dynamicCastFailure_SOURCE_TYPE_NULL(targetType, message);
+    }
+  } else if (targetType == nullptr) {
+      swift_dynamicCastFailure_TARGET_TYPE_NULL(sourceType, message);
+  }
+
   std::string sourceName = nameForMetadata(sourceType);
   std::string targetName = nameForMetadata(targetType);