MFC r317214:

Turn off llvm/clang's ENABLE_BACKTRACES setting, since it never worked
properly anyway. (Upstream has reorganized this somewhat in the mean
time, but for proper backtraces we would need llvm-symbolizer in base.)

MFC r317215:

Add function and data sections when building llvm, clang, lld and lldb,
and allow the linker to garbage collect them. This shaves off up to a
few MB from the final executables.