When did they start calling it Apple Clang? It was Apple LLVM Compiler for the longest time.
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
Clang is a compiler for C and C++ and Objective-C and Objective-C++.
LLVM is the underlying technology which is used by clang, but it's also used by other compilers (e.g. Swift uses LLVM, and some major JavaScript compilers as well).
Neither is owned by Apple - they're open source projects with contributors from Microsoft, Google, ARM, Intel, AMD and many others.