Make Node::addChild(NULL, ...) always assert.

We need this in order to gather relevant information when this situation
arises; it's clearly happening (e.g. rdar://86071019), but not often
enough for it to trigger in a build with assertions enabled.

rdar://89139049

Author: al45tair

lib/Demangling/Demangler.cpp

@@ -336,7 +336,7 @@ Node::iterator Node::end() const {
 }
 
 void Node::addChild(NodePointer Child, NodeFactory &Factory) {
-  assert(Child);
+  DEMANGLER_ALWAYS_ASSERT(Child, this);
   switch (NodePayloadKind) {
     case PayloadKind::None:
       InlineChildren[0] = Child;

lib/Demangling/DemanglerAssert.h

@@ -42,6 +42,13 @@
 
 #endif // SWIFT_RUNTIME || defined(NDEBUG)
 
+// DEMANGLER_ALWAYS_ASSERT() *always* fails the program, even in the runtime
+#define DEMANGLER_ALWAYS_ASSERT(expr, node)                                    \
+  do {                                                                         \
+    if (!(expr))                                                               \
+      swift::Demangle::failAssert(__FILE__, __LINE__, node, #expr);            \
+  } while (0)
+
 namespace swift {
 namespace Demangle {
 SWIFT_BEGIN_INLINE_NAMESPACE