[llvm] [ADT] Fix specialization of ValueIsPresent for PointerUnion (PR #121847)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-sve-vls` running on `linaro-g3-04` while building `llvm` at step 6 "build stage 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/143/builds/4665

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (build stage 1) failure: 'ninja' (failure)
...
[7883/8800] Building CXX object tools/flang/lib/Optimizer/Dialect/Support/CMakeFiles/FIRDialectSupport.dir/KindMapping.cpp.o
[7884/8800] Building CXX object tools/flang/lib/Evaluate/CMakeFiles/FortranEvaluate.dir/common.cpp.o
[7885/8800] Building CXX object tools/flang/lib/Evaluate/CMakeFiles/FortranEvaluate.dir/integer.cpp.o
[7886/8800] Building CXX object tools/flang/lib/Evaluate/CMakeFiles/FortranEvaluate.dir/real.cpp.o
[7887/8800] Building CXX object tools/flang/lib/Evaluate/CMakeFiles/FortranEvaluate.dir/static-data.cpp.o
[7888/8800] Building CXX object tools/flang/lib/Evaluate/CMakeFiles/FortranEvaluate.dir/target.cpp.o
[7889/8800] Building CXX object tools/flang/lib/Common/CMakeFiles/FortranCommon.dir/Version.cpp.o
[7890/8800] Building CXX object tools/flang/lib/Evaluate/CMakeFiles/FortranEvaluate.dir/complex.cpp.o
[7891/8800] Building CXX object tools/flang/lib/Evaluate/CMakeFiles/FortranEvaluate.dir/logical.cpp.o
[7892/8800] Building CXX object tools/flang/lib/Evaluate/CMakeFiles/FortranEvaluate.dir/host.cpp.o
command timed out: 1200 seconds without output running [b'ninja'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=3112.507224

```

</details>

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