Expose llvm.ptrmask intrinsic

[nikic]

LLVM recently added a ptrmask intrinsic, intended for use with tagged pointers: https://llvm.org/docs/LangRef.html#llvm-ptrmask-intrinsic

It allows masking off bits from a pointer without going through ptrtoint/inttoptr and breaking alias analysis in the process. We might want to expose this somehow...

[leonardo-m]

This could be a quite useful thing for Rust if handled well (at least used by a Cargo crate).

[Mark-Simulacrum]

This is now exposed on nightly: https://doc.rust-lang.org/nightly/std/primitive.pointer.html#method.mask. Since there's already a tracking issue (#98290), closing in favor of that.