Mysterious GDB hangs after clean & build
I ran into the same problem and was able to resolve it by compiling with GNU g++ and including level 3 GDB debugging info.
$ g++-10 -ggdb3 FILE.cpp

Addendum: adding my toolchain versions for posterity.
$ uname -a
Darwin manco.local 19.6.0 Darwin Kernel Version 19.6.0: Thu Oct 29 22:56:45 PDT 2020; root:xnu-6153.141.2.2~1/RELEASE_X86_64 x86_64 i386 MacBookPro15,2 Darwin

$ g++-10 --version
g++-10 (Homebrew GCC 10.2.0) 10.2.0
...

$ gdb --version
GNU gdb (GDB) 10.1
...