Add :BuildDir to select YouCompleteMe build directory

autoload/build.vim

@@ -0,0 +1,6 @@
+function! build#dir(dir) abort
+  let l:cwd = getcwd()
+  let $BUILD_DIR = l:cwd.'/'.a:dir
+  let g:ycm_clangd_args = ['--compile-commands-dir='.$BUILD_DIR]
+  YcmRestartServer
+endfunction

plugin/commands.vim

@@ -29,3 +29,6 @@ command! -nargs=+ -complete=file Debug call do#debug(<f-args>)
 
 " Find all TODO items in the current file and populate the location list
 command! TodoFile lvimgrep /todo/ %
+
+" Change build directory
+command! -nargs=1 -complete=dir BuildDir call build#dir(<f-args>)