Add infotable location info for thunks introduced by tag inference.

In some circumstances tag inference can introduce new (usually short lived) thunks when rewriting foo = Con x y to foo = case x of x' -> Con x' y.

Currently these don't get location info when using info table profiling. I think it shouldn't to terribly hard to give these thunks useful location info to make debugging easier.

Edit: @AndreasK says: See Note [Strict Field Invariant] and Note [How untagged pointers can end up in strict fields].