To generate debug symbols in clang, call it with the -g option.
The most simple solution would be to add a line to your top-level CMakeLists.txt:
add_compile_options(-g)
Add_compile_options
will unconditionally set the option for all targets in this directory and those below. It will do so for all configurations (it should already be set in debug and will do no harm if set twice).
To have more control for which targets and configurations to generate debug info, use target_compile_options
with a generator expression like the following:
target_compile_options(mytarget PRIVATE $<$<CONFIG:Release>:-g>)
This will work with GCC and clang but not with other compilers that use different options (most notably MSVC).