From what I can tell, it looks like the UBT expects Clang to be installed at C:\Program Files\LLVM. The UBT specifically verifies that the following file exists: C:\Program Files\LLVM\bin\clang-cl.exe
So I symbolically linked C:\Program Files\LLVM to the folder where I manually built LLVM/Clang. That seems to work, and I am able to generate the compilation database. It’s not my ideal solution, so if anyone has another suggestion, I’d be interested in hearing it!
Hope this helps!