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

[Sergei Barannikov via llvm-commits]

s-barannikov wrote:

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

Now it cannot select a specialization for `std::optional`, sigh.

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