[llvm] Fixed Demangle OutputBuffer (llvm#138564)

The [llvm-clang-x86_64-expensive-checks-win](https://lab.llvm.org/buildbot/#/builders/14/builds/3075)
buildbot has been broken by llvm#133249.
DemangleTests causes the exception 0x80000003 inside CRT in case of the debug build on Windows.

Author: slydiman

llvm/include/llvm/Demangle/Utility.h

@@ -136,6 +136,8 @@ class OutputBuffer {
 
   OutputBuffer &prepend(std::string_view R) {
     size_t Size = R.size();
+    if (!Size)
+      return *this;
 
     grow(Size);
     std::memmove(Buffer + Size, Buffer, CurrentPosition);