Add pointer masking convenience functions

This commit adds the following functions all of which have a signature
`pointer, usize -> pointer`:
- `<*mut T>::mask`
- `<*const T>::mask`
- `intrinsics::ptr_mask`

These functions are equivalent to `.map_addr(|a| a & mask)` but they
utilize `llvm.ptrmask` llvm intrinsic.

*masks your pointers*

compiler/rustc_span/src/symbol.rs

@@ -1112,6 +1112,7 @@ symbols! {
         ptr,
         ptr_guaranteed_eq,
         ptr_guaranteed_ne,
+        ptr_mask,
         ptr_null,
         ptr_null_mut,
         ptr_offset_from,