[llvm][formatters] Add LLDB formatter for llvm::PointerUnion (#175218)
We make use of the fact that the `PointerUnion` element is a
`PointerIntPair`, for which we have a synthetic provider already. We get
the `Int` portion of the pair (which is the index into the template
parameter pack of the union) to get the active type and the `Pointer`
portion of the pair to get the actual pointer value.
Before:
```
(lldb) (lldb) v -T z_float
(llvm::PointerUnion<Z *, float *>) z_float = {
(llvm::pointer_union_detail::PointerUnionMembers<llvm::PointerUnion<Z *, float *>, llvm::PointerIntPair<void *, 1, int, llvm::pointer_union_detail::PointerUnionUIntTraits<Z *, float *> >, 0, Z *, float *>) llvm::pointer_union_detail::PointerUnionMembers<llvm::PointerUnion<Z *, float *>, llvm::PointerIntPair<void *, 1, int, llvm::pointer_union_detail::PointerUnionUIntTraits<Z *, float *>, llvm::PointerIntPairInfo<void *, 1, llvm::pointer_union_detail::PointerUnionUIntTraits<Z *, float *> > >, 0, Z *, float *> = {
(llvm::pointer_union_detail::PointerUnionMembers<llvm::PointerUnion<Z *, float *>, llvm::PointerIntPair<void *, 1, int, llvm::pointer_union_detail::PointerUnionUIntTraits<Z *, float *> >, 1, float *>) llvm::pointer_union_detail::PointerUnionMembers<llvm::PointerUnion<Z *, float *>, llvm::PointerIntPair<void *, 1, int, llvm::pointer_union_detail::PointerUnionUIntTraits<Z *, float *>, llvm::PointerIntPairInfo<void *, 1, llvm::pointer_union_detail::PointerUnionUIntTraits<Z *, float *> > >, 1, float *> = {
(llvm::pointer_union_detail::PointerUnionMembers<llvm::PointerUnion<Z *, float *>, llvm::PointerIntPair<void *, 1, int, llvm::pointer_union_detail::PointerUnionUIntTraits<Z *, float *> >, 2>) llvm::pointer_union_detail::PointerUnionMembers<llvm::PointerUnion<Z *, float *>, llvm::PointerIntPair<void *, 1, int, llvm::pointer_union_detail::PointerUnionUIntTraits<Z *, float *>, llvm::PointerIntPairInfo<void *, 1, llvm::pointer_union_detail::PointerUnionUIntTraits<Z *, float *> > >, 2> = {
(llvm::PointerIntPair<void *, 1, int, llvm::pointer_union_detail::PointerUnionUIntTraits<Z *, float *> >) Val = {...}
}
}
}
}
```
After:
```
(llvm::PointerUnion<Z *, float *>) z_float = {
(float *) Pointer = 0x000000016fdfebb0
}
```
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