compiler: Add rustc_abi dependence to the compiler

Depend on rustc_abi in compiler crates that use it indirectly but have
not yet taken on that dependency, and are not entangled in my other PRs.
This leaves an "excise rustc_target" step after the dust settles.

compiler/rustc_mir_dataflow/src/drop_flag_effects.rs

@@ -1,5 +1,5 @@
+use rustc_abi::VariantIdx;
 use rustc_middle::mir::{self, Body, Location, Terminator, TerminatorKind};
-use rustc_target::abi::VariantIdx;
 use tracing::debug;
 
 use super::move_paths::{InitKind, LookupResult, MoveData, MovePathIndex};