Mark your build folder as excluded:
File > Project Structure > Modules > Sources > Mark as Excluded (red icon)
You can also just right click on your folder and select Mark Directory As > Excluded.
Excluded folders (shown as rootExcluded) are ones that IntelliJ IDEA “partially ignores”. Very limited coding assistance is provided for files in excluded folders. Classes contained in excluded folders don’t appear in code completion suggestion lists, references to such classes are shown in the editor as unresolved. When searching, IntelliJ IDEA doesn’t look in excluded folders, etc.
Source
Note: See the answer by Nader Hadji Ghanbari for another approach using Scopes.