Move drop elaboration infrastructure.

`rustc_mir_dataflow/src/elaborate_drops.rs` contains some infrastructure
used by a few MIR passes: the `elaborate_drop` function, the
`DropElaborator` trait, etc.

`rustc_mir_transform/src/elaborate_drops.rs` (same file name, different
crate) contains the `ElaborateDrops` pass. It relies on a lot of the
infrastructure from `rustc_mir_dataflow/src/elaborate_drops.rs`.

It turns out that the drop infrastructure is only used in
`rustc_mir_transform`, so this commit moves it there. (The only
exception is the small `DropFlagState` type, which is moved to the
existing `rustc_mir_dataflow/src/drop_flag_effects.rs`.) The file is
renamed from `rustc_mir_dataflow/src/elaborate_drops.rs` to
`rustc_mir_transform/src/elaborate_drop.rs` (with no trailing `s`)
because (a) the `elaborate_drop` function is the most important export,
and (b) `rustc_mir_transform/src/elaborate_drops.rs` already exists.

All the infrastructure pieces that used to be `pub` are now
`pub(crate)`, because they are now only used within
`rustc_mir_transform`.

compiler/rustc_mir_dataflow/src/drop_flag_effects.rs

@@ -3,7 +3,26 @@ use rustc_middle::mir::{self, Body, Location, Terminator, TerminatorKind};
 use tracing::debug;
 
 use super::move_paths::{InitKind, LookupResult, MoveData, MovePathIndex};
-use crate::elaborate_drops::DropFlagState;
+
+/// The value of an inserted drop flag.
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
+pub enum DropFlagState {
+    /// The tracked value is initialized and needs to be dropped when leaving its scope.
+    Present,
+
+    /// The tracked value is uninitialized or was moved out of and does not need to be dropped when
+    /// leaving its scope.
+    Absent,
+}
+
+impl DropFlagState {
+    pub fn value(self) -> bool {
+        match self {
+            DropFlagState::Present => true,
+            DropFlagState::Absent => false,
+        }
+    }
+}
 
 pub fn move_path_children_matching<'tcx, F>(
     move_data: &MoveData<'tcx>,