Conditionalize null ptr check when casting

Background: We've noticed a lot of problems from Obj-C APIs that returned null
even though they were declared to never do so.  These mismatches subvert Swift's
type system and can lead to hard-to-diagnose crashes much later in the program.
This fatal error was introduced into the primary casting function to help catch
such problems closer to the point where they occur so developers could more
easily identify and fix them.

However, there's been some concern about what this means for old binaries, so
we're considering a check here that would allow the old behavior in certain
cases yet to be determined.  This PR adds the framework for such a check.

Resolves rdar://72323929

Author: tbkka

stdlib/public/runtime/DynamicCast.cpp

@@ -110,6 +110,12 @@ extern "C" const StructDescriptor NOMINAL_TYPE_DESCR_SYM(Sh);
 /// Nominal type descriptor for Swift.String.
 extern "C" const StructDescriptor NOMINAL_TYPE_DESCR_SYM(SS);
 
+// If this returns `true`, then we will call `fatalError` when we encounter a
+// null reference in a storage locaation whose type does not allow null.
+static bool unexpectedNullIsFatal() {
+  return true;  // Placeholder for an upcoming check.
+}
+
 static HeapObject * getNonNullSrcObject(OpaqueValue *srcValue,
                                         const Metadata *srcType,
                                         const Metadata *destType) {
@@ -120,12 +126,21 @@ static HeapObject * getNonNullSrcObject(OpaqueValue *srcValue,
 
   std::string srcTypeName = nameForMetadata(srcType);
   std::string destTypeName = nameForMetadata(destType);
-  swift::fatalError(/* flags = */ 0,
-                    "Found unexpected null pointer value"
+  const char *msg = "Found unexpected null pointer value"
                     " while trying to cast value of type '%s' (%p)"
-                    " to '%s' (%p)\n",
-                    srcTypeName.c_str(), srcType,
-                    destTypeName.c_str(), destType);
+                    " to '%s' (%p)%s\n";
+  if (unexpectedNullIsFatal()) {
+    swift::fatalError(/* flags = */ 0, msg,
+                      srcTypeName.c_str(), srcType,
+                      destTypeName.c_str(), destType,
+                      "");
+  } else {
+    swift::warning(/* flags = */ 0, msg,
+                   srcTypeName.c_str(), srcType,
+                   destTypeName.c_str(), destType,
+                   ": Continuing with null object, but expect problems later.");
+  }
+  return object;
 }
 
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