As far back as 2007 it was called clang. I'm not sure if it was called clang internally before that. See the seventh slide in the linked presentation.
https://web.archive.org/web/20190403123249/http://llvm.org/d...
https://web.archive.org/web/20190403123249/http://llvm.org/d...
MacPorts suggests that the --version string changed in Xcode 11 with macOS 10.14 Mojave, but I don't know if the Clang name was reflected in the Xcode GUI at that point. https://trac.macports.org/wiki/XcodeVersionInfo