Remove unnecessary lifetimes in dataflow structs.

There are four related dataflow structs: `MaybeInitializedPlaces`,
`MaybeUninitializedPlaces`, and `EverInitializedPlaces`,
`DefinitelyInitializedPlaces`. They all have a `&Body` and a
`&MoveData<'tcx>` field. The first three use different lifetimes for the
two fields, but the last one uses the same lifetime for both.

This commit changes the first three to use the same lifetime, removing
the need for one of the lifetimes. Other structs that also lose a
lifetime as a result of this are `LivenessContext`, `LivenessResults`,
`InitializationData`.

It then does similar things in various other structs.

compiler/rustc_borrowck/src/used_muts.rs

@@ -7,7 +7,7 @@ use tracing::debug;
 
 use crate::MirBorrowckCtxt;
 
-impl<'tcx> MirBorrowckCtxt<'_, '_, '_, 'tcx> {
+impl<'tcx> MirBorrowckCtxt<'_, '_, 'tcx> {
     /// Walks the MIR adding to the set of `used_mut` locals that will be ignored for the purposes
     /// of the `unused_mut` lint.
     ///
@@ -46,13 +46,13 @@ impl<'tcx> MirBorrowckCtxt<'_, '_, '_, 'tcx> {
 
 /// MIR visitor for collecting used mutable variables.
 /// The 'visit lifetime represents the duration of the MIR walk.
-struct GatherUsedMutsVisitor<'visit, 'a, 'mir, 'infcx, 'tcx> {
+struct GatherUsedMutsVisitor<'a, 'b, 'infcx, 'tcx> {
     temporary_used_locals: FxIndexSet<Local>,
-    never_initialized_mut_locals: &'visit mut FxIndexSet<Local>,
-    mbcx: &'visit mut MirBorrowckCtxt<'a, 'mir, 'infcx, 'tcx>,
+    never_initialized_mut_locals: &'a mut FxIndexSet<Local>,
+    mbcx: &'a mut MirBorrowckCtxt<'b, 'infcx, 'tcx>,
 }
 
-impl GatherUsedMutsVisitor<'_, '_, '_, '_, '_> {
+impl GatherUsedMutsVisitor<'_, '_, '_, '_> {
     fn remove_never_initialized_mut_locals(&mut self, into: Place<'_>) {
         // Remove any locals that we found were initialized from the
         // `never_initialized_mut_locals` set. At the end, the only remaining locals will
@@ -64,7 +64,7 @@ impl GatherUsedMutsVisitor<'_, '_, '_, '_, '_> {
     }
 }
 
-impl<'tcx> Visitor<'tcx> for GatherUsedMutsVisitor<'_, '_, '_, '_, 'tcx> {
+impl<'tcx> Visitor<'tcx> for GatherUsedMutsVisitor<'_, '_, '_, 'tcx> {
     fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
         debug!("visit_terminator: terminator={:?}", terminator);
         match &terminator.kind {