Refactor: VariantIdx::from_u32(0) -> FIRST_VARIANT

Since structs are always `VariantIdx(0)`, there's a bunch of files where the only reason they had `VariantIdx` or `vec::Idx` imported at all was to get the first variant.

So this uses a constant for that, and adds some doc-comments to `VariantIdx` while I'm there, since it doesn't have any today.

compiler/rustc_const_eval/src/const_eval/valtrees.rs

@@ -8,7 +8,7 @@ use crate::interpret::{
 use crate::interpret::{MPlaceTy, Value};
 use rustc_middle::ty::{self, ScalarInt, Ty, TyCtxt};
 use rustc_span::source_map::DUMMY_SP;
-use rustc_target::abi::{Align, VariantIdx};
+use rustc_target::abi::{Align, VariantIdx, FIRST_VARIANT};
 
 #[instrument(skip(ecx), level = "debug")]
 fn branches<'tcx>(
@@ -412,7 +412,7 @@ fn valtree_into_mplace<'tcx>(
 
                         let inner_ty = match ty.kind() {
                             ty::Adt(def, substs) => {
-                                def.variant(VariantIdx::from_u32(0)).fields[i].ty(tcx, substs)
+                                def.variant(FIRST_VARIANT).fields[i].ty(tcx, substs)
                             }
                             ty::Tuple(inner_tys) => inner_tys[i],
                             _ => bug!("unexpected unsized type {:?}", ty),