Fix #1279 - Run cppcheck differently when modified

cppcheck is now run without the --project option and from the buffer's
directory instead when the buffer has been modified. Saving the buffer
will get results by linting the project instead.

autoload/ale/handlers/cppcheck.vim

@@ -1,5 +1,46 @@
 " Description: Handle errors for cppcheck.
 
+function! ale#handlers#cppcheck#GetCdCommand(buffer) abort
+    let [l:dir, l:json_path] = ale#c#FindCompileCommands(a:buffer)
+    let l:cd_command = !empty(l:dir) ? ale#path#CdString(l:dir) : ''
+
+    return l:cd_command
+endfunction
+
+function! ale#handlers#cppcheck#GetBufferPathIncludeOptions(buffer) abort
+    let l:buffer_path_include = ''
+
+    " Get path to this buffer so we can include it into cppcheck with -I
+    " This could be expanded to get more -I directives from the compile
+    " command in compile_commands.json, if it's found.
+    let l:buffer_path = fnamemodify(bufname(a:buffer), ':p:h')
+    let l:buffer_path_include = ' -I' . ale#Escape(l:buffer_path)
+
+    return l:buffer_path_include
+endfunction
+
+function! ale#handlers#cppcheck#GetCompileCommandsOptions(buffer) abort
+    " If the current buffer is modified, using compile_commands.json does no
+    " good, so include the file's directory instead. It's not quite as good as
+    " using --project, but is at least equivalent to running cppcheck on this
+    " file manually from the file's directory.
+    let l:modified = getbufvar(a:buffer, '&modified')
+
+    if l:modified
+        return ''
+    endif
+
+    " Search upwards from the file for compile_commands.json.
+    "
+    " If we find it, we'll `cd` to where the compile_commands.json file is,
+    " then use the file to set up import paths, etc.
+    let [l:dir, l:json_path] = ale#c#FindCompileCommands(a:buffer)
+
+    return !empty(l:json_path)
+    \   ? '--project=' . ale#Escape(l:json_path[len(l:dir) + 1: ])
+    \   : ''
+endfunction
+
 function! ale#handlers#cppcheck#HandleCppCheckFormat(buffer, lines) abort
     " Look for lines like the following.
     "