[clang][docs] Add note about using `-flto` with `-g` on macOS

If -Wl,object_path_lto,<lto-filename>.o is not passed at link time
when compiling and linking in separate steps with -flto and -g, the
temporary file used for Link Time Optimization is deleted by the linker,
so the executable is missing debug symbols and can't be easily debugged,
and dsymutil can't be run.

Document this behaviour.

Differential revision: https://reviews.llvm.org/D82733