LLDB is a high performance debugger, and the default debugger in Xcode on macOS.
After compiling with ./configure --enable-debug:
lldb src/qt/bitcoin-qt
(lldb) target create "src/qt/bitcoin-qt"
Current executable set to 'src/qt/bitcoin-qt' (x86_64).
(lldb) run
...src/qt/bitcoin-qt -- -testnet
(lldb) target create "src/qt/bitcoin-qt"
Current executable set to 'src/qt/bitcoin-qt' (x86_64).
(lldb) settings set -- target.run-args "-testnet"
(lldb) run
...After a crash, pass bt.