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

[A. Jiang via llvm-commits]

frederick-vs-ja wrote:

Looks like that we need to run libc++abi tests when changes are made in `llvm/Demangle`, per [`itanium_demangle_matches_llvm.sh.test`](https://github.com/llvm/llvm-project/blob/bfcd67c34731aae4800ccfc897ebb3a679ea9136/libcxxabi/test/itanium_demangle_matches_llvm.sh.test).

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