Fix memory leak in MicrosoftDemangleNodes's Node::toString

[teemperor]

Summary

The buffer we turn into a std::string here is malloc'd and should be
free'd before we return from this function.

Follow up to LLDB leak fixes such as D100806.

[mstorsjo]

Oops, and thanks for fixing it!

llvm/lib/Demangle/MicrosoftDemangleNodes.cpp

static void outputCallingConvention(OutputStream &OS, CallingConv CC) {
   }
 }
 
 std::string Node::toString(OutputFlags Flags) const {
   OutputStream OS;
   initializeOutputStream(nullptr, nullptr, OS, 1024);
   this->output(OS, Flags);
   OS << '\0';
-  return {OS.getBuffer()};
+  std::string Owned(OS.getBuffer());
+  std::free(OS.getBuffer());
+  return Owned;
 }
 
 void PrimitiveTypeNode::outputPre(OutputStream &OS, OutputFlags Flags) const {
   switch (PrimKind) {
     OUTPUT_ENUM_CLASS_VALUE(PrimitiveKind, Void, "void");
     OUTPUT_ENUM_CLASS_VALUE(PrimitiveKind, Bool, "bool");
     OUTPUT_ENUM_CLASS_VALUE(PrimitiveKind, Char, "char");
     OUTPUT_ENUM_CLASS_VALUE(PrimitiveKind, Schar, "signed char");