Fix an OOB read in the demangler (swiftlang#31793) (swiftlang#31973)

A malformed mangled name that ends in a truncated symbolic
reference could trigger a read beyond the end of the name.
This is because the code that grabs the next four bytes
bypasses the existing bounds checks.  Insert an explicit
bounds check to guard against this.

Author: tbkka

include/swift/Demangling/Demangler.h

@@ -564,8 +564,7 @@ class Demangler : public NodeFactory {
   NodePointer demangleValueWitness();
 
   NodePointer demangleTypeMangling();
-  NodePointer demangleSymbolicReference(unsigned char rawKind,
-                                        const void *at);
+  NodePointer demangleSymbolicReference(unsigned char rawKind);
 
   bool demangleBoundGenerics(Vector<NodePointer> &TypeListList,
                              NodePointer &RetroactiveConformances);

lib/Demangling/Demangler.cpp

@@ -677,12 +677,14 @@ NodePointer Demangler::demangleTypeMangling() {
   return TypeMangling;
 }
 
-NodePointer Demangler::demangleSymbolicReference(unsigned char rawKind,
-                                                 const void *at) {
+NodePointer Demangler::demangleSymbolicReference(unsigned char rawKind) {
   // The symbolic reference is a 4-byte machine integer encoded in the following
   // four bytes.
+  if (Pos + 4 > Text.size())
+    return nullptr;
+  const void *at = Text.data() + Pos;
   int32_t value;
-  memcpy(&value, Text.data() + Pos, 4);
+  memcpy(&value, at, 4);
   Pos += 4;
 
   // Map the encoded kind to a specific kind and directness.
@@ -734,7 +736,7 @@ NodePointer Demangler::demangleOperator() {
       goto recur;
     case 1: case 2:   case 3:   case 4:   case 5: case 6: case 7: case 8:
     case 9: case 0xA: case 0xB: case 0xC:
-      return demangleSymbolicReference((unsigned char)c, Text.data() + Pos);
+      return demangleSymbolicReference((unsigned char)c);
     case 'A': return demangleMultiSubstitutions();
     case 'B': return demangleBuiltinType();
     case 'C': return demangleAnyGenericType(Node::Kind::Class);