[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.

I added IsNullable check back to work around the issue.


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