Auto merge of #14184 - lowr:feat/trait-alias-def, r=Veykril

Handle trait alias definitions

Part of #2773

This PR adds a bunch of structs and enum variants for trait aliases. Trait aliases should be handled as an independent item because they are semantically distinct from traits.

I basically started by adding `TraitAlias{Id, Loc}` to `hir_def::item_tree` and iterated adding necessary stuffs until compiler stopped complaining what's missing. Let me know if there's still anything I need to add.

I'm opening up this PR for early review and stuff. I'm planning to add tests for IDE functionalities in this PR, but not type-related support, for which I put FIXME notes.

crates/ide-db/src/lib.rs

@@ -191,6 +191,7 @@ pub enum SymbolKind {
     Struct,
     ToolModule,
     Trait,
+    TraitAlias,
     TypeAlias,
     TypeParam,
     Union,
@@ -221,6 +222,7 @@ impl From<FileSymbolKind> for SymbolKind {
             FileSymbolKind::Static => SymbolKind::Static,
             FileSymbolKind::Struct => SymbolKind::Struct,
             FileSymbolKind::Trait => SymbolKind::Trait,
+            FileSymbolKind::TraitAlias => SymbolKind::TraitAlias,
             FileSymbolKind::TypeAlias => SymbolKind::TypeAlias,
             FileSymbolKind::Union => SymbolKind::Union,
         }