clangtidy supports build_dir and cpp_clangtidy_options at the same time

The `clang-tidy` command itself supports both `-p` and `--` at the same time. Checked in Debian version 9.0-49.1.

However the code snippet
https://github.com/dense-analysis/ale/blob/master/ale_linters/cpp/clangtidy.vim#L22-L26
```vim
if empty(l:build_dir)
let l:options = ale#Var(a:buffer, 'cpp_clangtidy_options')
let l:cflags = ale#c#GetCFlags(a:buffer, a:output)
let l:options .= !empty(l:options) ? ale#Pad(l:cflags) : l:cflags
endif
```
indicates that if `build_dir` provided, `cpp_clangtidy_options` will be ignored. Could you help support both at the same time?