Your switch from downloading Clang builds to building clang from source snapshots is broke and busted.
I built ue5-main and 5.3 branches, then a few hours later pulled the latest from github, and the GenerateProjectFiles.sh script is now saying:
“Exception while generating include data for UnrealEditor: Platform Linux is not a valid platform to build. Check that the SDK is installed properly.”
I just now tried doing a clean checkout and build of the 5.3 branch and Setup.sh STILL downloads LLVM/Clang 15 and makes no attempt to obtain and build LLVM/Clang 16.