Rename Rptr to Ref in AST and HIR

The name makes a lot more sense, and `ty::TyKind` calls it `Ref` already
as well.

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -1209,7 +1209,7 @@ fn get_mut_span_in_struct_field<'tcx>(
         // Now we're dealing with the actual struct that we're going to suggest a change to,
         // we can expect a field that is an immutable reference to a type.
         && let hir::Node::Field(field) = node
-        && let hir::TyKind::Rptr(lt, hir::MutTy { mutbl: hir::Mutability::Not, ty }) = field.ty.kind
+        && let hir::TyKind::Ref(lt, hir::MutTy { mutbl: hir::Mutability::Not, ty }) = field.ty.kind
     {
         return Some(lt.ident.span.between(ty.span));
     }