Shrink hir::def::Res.

`Res::SelfTy` currently has two `Option`s. When the second one is `Some`
the first one is never consulted. So we can split it into two variants,
`Res::SelfTyParam` and `Res::SelfTyAlias`, reducing the size of `Res`
from 24 bytes to 12. This then shrinks `hir::Path` and
`hir::PathSegment`, which are the HIR types that take up the most space.

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -1010,7 +1010,8 @@ impl<'a, 'b> BuildReducedGraphVisitor<'a, 'b> {
                 _,
             )
             | Res::Local(..)
-            | Res::SelfTy { .. }
+            | Res::SelfTyParam { .. }
+            | Res::SelfTyAlias { .. }
             | Res::SelfCtor(..)
             | Res::Err => bug!("unexpected resolution: {:?}", res),
         }