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/mir/mod.rs

@@ -2014,7 +2014,7 @@ impl<'tcx> Rvalue<'tcx> {
                 | CastKind::IntToFloat
                 | CastKind::FnPtrToPtr
                 | CastKind::PtrToPtr
-                | CastKind::Pointer(_)
+                | CastKind::PointerCoercion(_)
                 | CastKind::PointerFromExposedAddress
                 | CastKind::DynStar
                 | CastKind::Transmute,