2020-05-27 Fixing a slow build in Xcode

If your Xcode project isn't compiling as fast as you expect, open the build settings of target that you're building. Add the following to the "Other Swift Flags" setting:

    -Xfrontend -warn-long-expression-type-checking=100

This makes the compiler emit warnings when it takes more than 100 ms to type-check an expression.