Implement IR generation for CHARSET_ACTION_SCAN, CHARSET_ACTION_ACCEPT and CHARSET_ACTION_COMPLEMENT

Add --disable-llvm-backend/--enable-llvm-backend build-time options, and move llvm.c to src/backends/llvm/ subdir

Avoid redundant charset copy (and leak) in h_llvm_build_charset_exec_plan_impl_alloc(); h_llvm_build_charset_exec_plan_impl() already knows to do this

Only try to split if we have more than one char in range, and be sure to free the charset if h_llvm_find_best_split() fails

CEP nodes can have cost 0 now; we break ties against unnecessary complements by testing for that case last