[libc++abi] Use = default; for OutputBuffer's destructor

This mirrors the change in 4eed683 to
libcxxabi, which is necessary for fixing CI failure.

Author: frederick-vs-ja

libcxxabi/src/demangle/Utility.h

@@ -81,7 +81,7 @@ class OutputBuffer {
   OutputBuffer(const OutputBuffer &) = delete;
   OutputBuffer &operator=(const OutputBuffer &) = delete;
 
-  virtual ~OutputBuffer() {}
+  virtual ~OutputBuffer() = default;
 
   operator std::string_view() const {
     return std::string_view(Buffer, CurrentPosition);