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_save_analysis/src/lib.rs

@@ -740,7 +740,8 @@ impl<'tcx> SaveContext<'tcx> {
                 _,
             )
             | Res::PrimTy(..)
-            | Res::SelfTy { .. }
+            | Res::SelfTyParam { .. }
+            | Res::SelfTyAlias { .. }
             | Res::ToolMod
             | Res::NonMacroAttr(..)
             | Res::SelfCtor(..)
@@ -805,7 +806,7 @@ impl<'tcx> SaveContext<'tcx> {
 
     fn lookup_def_id(&self, ref_id: hir::HirId) -> Option<DefId> {
         match self.get_path_res(ref_id) {
-            Res::PrimTy(_) | Res::SelfTy { .. } | Res::Err => None,
+            Res::PrimTy(_) | Res::SelfTyParam { .. } | Res::SelfTyAlias { .. } | Res::Err => None,
             def => def.opt_def_id(),
         }
     }