A little update (since I have not found a reliable way how to upgrade the existing High Sierra pre 10.13.5 in VirtualBox) is to follow the exact same steps as mentioned here
https://astr0baby.wordpress.com/2018/04/22/installing-high-sierra-10-13-4-in-virtualbox-5-x/
With these exceptions
Download MacOS 10.13.5 iso from here -> https://drive.google.com/file/d/1_wnCdMKB4_GQM9dtA4AOHxZ21NCGFNm9/view
When you get the UEFI prompt follow the same steps, but boot via boot command only once in the correct UEFI directory as seen in the screenshot below
This will install High Sierra 10.13.5
And fresh xcode gcc
