Move mir::Field → abi::FieldIdx

The first PR for https://github.com/rust-lang/compiler-team/issues/606

This is just the move-and-rename, because it's plenty big-and-bitrotty already.  Future PRs will start using `FieldIdx` more broadly, and concomitantly removing `FieldIdx::new`s.

compiler/rustc_borrowck/src/path_utils.rs

@@ -7,8 +7,9 @@ use crate::BorrowIndex;
 use crate::Upvar;
 use rustc_data_structures::graph::dominators::Dominators;
 use rustc_middle::mir::BorrowKind;
-use rustc_middle::mir::{BasicBlock, Body, Field, Location, Place, PlaceRef, ProjectionElem};
+use rustc_middle::mir::{BasicBlock, Body, Location, Place, PlaceRef, ProjectionElem};
 use rustc_middle::ty::TyCtxt;
+use rustc_target::abi::FieldIdx;
 
 /// Returns `true` if the borrow represented by `kind` is
 /// allowed to be split into separate Reservation and
@@ -148,7 +149,7 @@ pub(crate) fn is_upvar_field_projection<'tcx>(
     upvars: &[Upvar<'tcx>],
     place_ref: PlaceRef<'tcx>,
     body: &Body<'tcx>,
-) -> Option<Field> {
+) -> Option<FieldIdx> {
     let mut place_ref = place_ref;
     let mut by_ref = false;