Rename ImplItem to ImplBlock

rustc uses the name ImplItem for items in impls, not the impl {} block itself,
which could lead to confusion.

crates/ra_lsp_server/src/conv.rs

@@ -44,7 +44,7 @@ impl Conv for SyntaxKind {
             SyntaxKind::TYPE_DEF => SymbolKind::TypeParameter,
             SyntaxKind::STATIC_DEF => SymbolKind::Constant,
             SyntaxKind::CONST_DEF => SymbolKind::Constant,
-            SyntaxKind::IMPL_ITEM => SymbolKind::Object,
+            SyntaxKind::IMPL_BLOCK => SymbolKind::Object,
             _ => SymbolKind::Variable,
         }
     }