You can debug release builds with gcc/clang just fine. They don't generate debug information by default, but you can always request it ("-O3 -g" is a perfectly fine combination of flags).

Not really, because some optimizations get the step through and such rather confusing.

VC++ dynamic debugging pretends the code motion, inlining and similar optimizations aren't there and maps back to the original code as written.

Unless this has been improved for gdb,lldb.