Adding Regular Expression information to the project.build.cs no longer works

This worked in previous releases, but changed around 4.27.2 and Early Access. This prevents users from using C# regular expressions.

1. Create a C++ project named MyProject
2. Open MyProject.build.cs
3. Paste this line on at the top: 

   using System.Text.RegularExpressions;

1. Above '// Uncomment if you...' paste this line: 

   Regex rgx = new Regex("");

1. Build the project.

Expected Results:
Project builds without error

Actual Results:
Project fails to build.