[clang] [llvm] Reland "[llvm][ADT] Refactor PointerUnion to use PunnedPointer. NFC." (PR #188242)

[Michael Buch via cfe-commits]

Michael137 wrote:

FYI this broke the LLDB data-formatter for pointer-union:
```
09:48:44  /Users/ec2-user/jenkins/workspace/llvm.org/as-lldb-cmake-os-verficiation/lldb-build/projects/cross-project-tests/debuginfo-tests/llvm-prettyprinters/lldb/Output/pointer-union.test.tmp/checks:18:15: error: CHECK-NEXT: expected string not found in input
09:48:44  # CHECK-NEXT: (float *) Pointer = [[PTR_F]]
09:48:44                ^
09:48:44  <stdin>:32:47: note: scanning from here
09:48:44  (llvm::PointerUnion<Z *, float *>) z_float = {}
09:48:44                                                ^
09:48:44  <stdin>:32:47: note: with "PTR_F" equal to "0x000000016fdff088"
09:48:44  (llvm::PointerUnion<Z *, float *>) z_float = {}
09:48:44                                                ^
09:48:44  <stdin>:41:24: note: possible intended match here
09:48:44   (llvm::PointerUnion<Z *, float *>) Pointer = {}
09:48:44                         ^
09:48:44  
09:48:44  Input file: <stdin>
```

https://github.com/llvm/llvm-project/pull/188242