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_middle/src/query/keys.rs

@@ -12,6 +12,7 @@ use rustc_hir::hir_id::{HirId, OwnerId};
 use rustc_query_system::query::{DefaultCacheSelector, SingleCacheSelector, VecCacheSelector};
 use rustc_span::symbol::{Ident, Symbol};
 use rustc_span::{Span, DUMMY_SP};
+use rustc_target::abi::FieldIdx;
 
 /// Placeholder for `CrateNum`'s "local" counterpart
 #[derive(Copy, Clone, Debug)]
@@ -332,7 +333,7 @@ impl<'tcx> Key for (ty::ParamEnv<'tcx>, ty::PolyTraitRef<'tcx>) {
     }
 }
 
-impl<'tcx> Key for (ty::Const<'tcx>, mir::Field) {
+impl<'tcx> Key for (ty::Const<'tcx>, FieldIdx) {
     type CacheSelector = DefaultCacheSelector<Self>;
 
     fn default_span(&self, _: TyCtxt<'_>) -> Span {