Structure defining the extent of one object on the stack that HWASAN needs
to tag in the corresponding shadow stack space.
The range this object spans on the stack is between `untagged_base +
nearest_offset` and `untagged_base + farthest_offset`.
`tagged_base` is an rtx containing the same value as `untagged_base` but
with a random tag stored in the top byte. We record both `untagged_base`
and `tagged_base` so that `hwasan_emit_prologue` can use both without having
to emit RTL into the instruction stream to re-calculate one from the other.
(`hwasan_emit_prologue` needs to use both bases since the
__hwasan_tag_memory call it emits uses an untagged value, and it calculates
the tag to store in shadow memory based on the tag_offset plus the tag in
tagged_base).
