rustc: Harmonize DefKind and DefPathData

`DefPathData::(ClosureExpr,ImplTrait)` are renamed to match `DefKind::(Closure,OpaqueTy)`.

`DefPathData::ImplTraitAssocTy` is replaced with `DefPathData::TypeNS(kw::Empty)` because both correspond to `DefKind::AssocTy`.
It's possible that introducing `(DefKind,DefPathData)::AssocOpaqueTy` could be a better solution, but that would be a much more invasive change.

Const generic parameters introduced for effects are moved from `DefPathData::TypeNS` to `DefPathData::ValueNS`, because constants are values.

`DefPathData` is no longer passed to `create_def` functions to avoid redundancy.

compiler/rustc_symbol_mangling/src/v0.rs

@@ -770,17 +770,16 @@ impl<'tcx> Printer<'tcx> for SymbolMangler<'tcx> {
             // Uppercase categories are more stable than lowercase ones.
             DefPathData::TypeNs(_) => 't',
             DefPathData::ValueNs(_) => 'v',
-            DefPathData::ClosureExpr => 'C',
+            DefPathData::Closure => 'C',
             DefPathData::Ctor => 'c',
             DefPathData::AnonConst => 'k',
-            DefPathData::ImplTrait => 'i',
+            DefPathData::OpaqueTy => 'i',
 
             // These should never show up as `path_append` arguments.
             DefPathData::CrateRoot
             | DefPathData::Use
             | DefPathData::GlobalAsm
             | DefPathData::Impl
-            | DefPathData::ImplTraitAssocTy
             | DefPathData::MacroNs(_)
             | DefPathData::LifetimeNs(_) => {
                 bug!("symbol_names: unexpected DefPathData: {:?}", disambiguated_data.data)