Simplify the code slightly.

I was trying to use fatalError() here, but that is in libswiftRuntime,
which isn't linked into everything that has the demangler code in it.
The upshot is that we'd have to touch a lot of other projects to use
fatalError().

rdar://89139049

Author: al45tair

lib/Demangling/Demangler.cpp

@@ -93,12 +93,13 @@ static bool isRequirement(Node::Kind kind) {
 
 void swift::Demangle::failAssert(const char *file, unsigned line,
                                  NodePointer node, const char *expr) {
-  fprintf(stderr, "%s:%u: assertion failed for Node %p: %s\n", file, line, node,
-          expr);
-  if (node) {
-    fprintf(stderr, "%s:%u: Node %p is:\n", file, line, node);
-    node->dump();
-  }
+  std::string treeStr = getNodeTreeAsString(node);
+
+  fprintf(stderr,
+          "%s:%u: assertion failed for Node %p: %s\n"
+          "%s:%u: Node %p is:\n%s\n",
+          file, line, node, expr,
+          file, line, node, treeStr.c_str());
   abort();
 }