Basic support for .Self within :! bindings and where expressions. (#1311)

`.Self` is modeled as a new kind of expression, `DotSelfExpression`. Name resolution associates each occurrence of a `DotSelfExpression` with a generic binding, much like for an `IdentifierExpression`.

Both `:!` bindings and `where` expressions bring `.Self` into scope. The tentative intent is that if there are multiple `.Self`s in scope and they refer to different bindings, the result of using a `.Self` expression is an ambiguity error, but that is not implemented in this patch. Instead, like for `IdentifierExpression`s, we find the innermost enclosing definition of `.Self`.

`.Foo` expressions are rewritten to `.Self.Foo`, but this isn't enough to make them do anything useful, because associated constants aren't supported in general yet.

explorer/syntax/BUILD

@@ -81,7 +81,6 @@ cc_library(
         "//explorer/ast:declaration",
         "//explorer/ast:expression",
         "//explorer/ast:paren_contents",
-        "//explorer/ast:pattern",
         "//explorer/ast:value_category",
         "//explorer/common:arena",
         "//explorer/common:error_builders",

explorer/syntax/parser.ypp

@@ -301,6 +301,15 @@ api_or_impl:
 primary_expression:
   identifier
     { $$ = arena->New<IdentifierExpression>(context.source_loc(), $1); }
+| designator
+    {
+      // `.Foo` is rewritten to `.Self.Foo`.
+      $$ = arena->New<SimpleMemberAccessExpression>(
+          context.source_loc(),
+          arena->New<DotSelfExpression>(context.source_loc()), $1);
+    }
+| PERIOD SELF
+    { $$ = arena->New<DotSelfExpression>(context.source_loc()); }
 | integer_literal
     { $$ = arena->New<IntLiteral>(context.source_loc(), $1); }
 | string_literal
@@ -544,7 +553,11 @@ where_clause_list:
 ;
 where_expression:
   type_expression WHERE where_clause_list
-    { $$ = arena->New<WhereExpression>(context.source_loc(), $1, $3); }
+    {
+      auto* self =
+          arena -> New<GenericBinding>(context.source_loc(), ".Self", $1);
+      $$ = arena->New<WhereExpression>(context.source_loc(), self, $3);
+    }
 ;
 statement_expression:
   ref_deref_expression