I want to try and answer this question and also for those that come later that installed Mojave and had an older Xcode version installed. The process is the same minus that if you have Xcode 10, you don't need to install it again from my understanding.
The folder is hidden and was not accessible even with a sudo find
command, however, I figured out that copying the absolute path and then pasting it in your browser address bar will give you the download and access to the folder that you need to fix brew install.
I posted a video here- https://youtu.be/GfnhSirTCGM that walks you thru part of my steps above. After you do the SDK headers download, its likely that unless you already are running Xcode 9.4, you will need to visit, https://developer.apple.com/download/more/ and look for Xcode 9.4 to install. Also grab the 10.14 command line tools as well. The lesson that should be learned here is that Apple calls it "Beta" for a reason and if you are a developer that knows the pains of setup, it is a bad idea. I hope that I was helpful! The last thing is, INSTALL XCODE THEN COMMAND LINE TOOLS-IN THAT ORDER OR IT ERRORS OUT! and then we do this all over again. Cheers!
It appears as though Apple has released a patch for the bug and it is available through the system preferences. 14 July 2018
Broken command line tools:

Xcode Location on the Apple developers website:

Location of command line tools:

Do not install Xcode from the Mac App Store because a different version is required!

Command line tools 10.14 update:
