[llvm] [llvm] Use "= default" (NFC) (PR #166088)

[A. Jiang via llvm-commits]

frederick-vs-ja wrote:

This caused CI failures in libcxxabi test.

```
  # .---command stdout------------
  # | *** /home/gha/actions-runner/_work/llvm-project/llvm-project/build/generic-cxx03/libcxxabi/test/Output/itanium_demangle_matches_llvm.sh.test.dir/t.tmp.libcxxabi_utility
  # | --- /home/gha/actions-runner/_work/llvm-project/llvm-project/build/generic-cxx03/libcxxabi/test/Output/itanium_demangle_matches_llvm.sh.test.dir/t.tmp.llvm_utility
  # | ***************
  # | *** 79,85 ****
  # |     OutputBuffer(const OutputBuffer &) = delete;
  # |     OutputBuffer &operator=(const OutputBuffer &) = delete;
  # |   
  # | !   virtual ~OutputBuffer() {}
  # |   
  # |     operator std::string_view() const {
  # |       return std::string_view(Buffer, CurrentPosition);
  # | --- 79,85 ----
  # |     OutputBuffer(const OutputBuffer &) = delete;
  # |     OutputBuffer &operator=(const OutputBuffer &) = delete;
  # |   
  # | !   virtual ~OutputBuffer() = default;
  # |   
  # |     operator std::string_view() const {
  # |       return std::string_view(Buffer, CurrentPosition);
  # `-----------------------------
  # error: command failed with exit status: 1
```

We probably also need to change `libcxxabi/src/demangle/Utility.h`.

https://github.com/llvm/llvm-project/pull/166088