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_const_eval/src/transform/validate.rs

@@ -650,7 +650,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                     // FIXME: Add Checks for these
                     CastKind::PointerFromExposedAddress
                     | CastKind::PointerExposeAddress
-                    | CastKind::Pointer(_) => {}
+                    | CastKind::PointerCoercion(_) => {}
                     CastKind::IntToInt | CastKind::IntToFloat => {
                         let input_valid = op_ty.is_integral() || op_ty.is_char() || op_ty.is_bool();
                         let target_valid = target_type.is_numeric() || target_type.is_char();