Rename adjustment::PointerCast and variants using it to PointerCoercion

It makes it sound like the `ExprKind` and `Rvalue` are supposed to represent all pointer related
casts, when in reality their just used to share a some enum variants. Make it clear there these
are only coercion to make it clear why only some pointer related "casts" are in the enum.

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -332,7 +332,7 @@ TrivialTypeTraversalAndLiftImpls! {
     crate::ty::IntVarValue,
     crate::ty::ParamConst,
     crate::ty::ParamTy,
-    crate::ty::adjustment::PointerCast,
+    crate::ty::adjustment::PointerCoercion,
     crate::ty::RegionVid,
     crate::ty::UniverseIndex,
     crate::ty::Variance,