[llvm] [ADT] Specialize ValueIsPresent for PointerUnion (PR #121847)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

unittests/Support/CMakeFiles/SupportTests.dir/Casting.cpp.o fails to build in pre-commit CI.

https://github.com/llvm/llvm-project/pull/121847