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/ast/expression.cpp

@@ -7,6 +7,7 @@
 #include <map>
 #include <optional>
 
+#include "explorer/ast/pattern.h"
 #include "explorer/common/arena.h"
 #include "explorer/common/error_builders.h"
 #include "llvm/ADT/StringExtras.h"
@@ -184,7 +185,7 @@ void Expression::Print(llvm::raw_ostream& out) const {
     }
     case ExpressionKind::WhereExpression: {
       const auto& where = cast<WhereExpression>(*this);
-      out << where.base() << " where ";
+      out << where.self_binding().type() << " where ";
       llvm::ListSeparator sep(" and ");
       for (const WhereClause* clause : where.clauses()) {
         out << sep << *clause;
@@ -213,6 +214,7 @@ void Expression::Print(llvm::raw_ostream& out) const {
       break;
     }
     case ExpressionKind::IdentifierExpression:
+    case ExpressionKind::DotSelfExpression:
     case ExpressionKind::IntLiteral:
     case ExpressionKind::BoolLiteral:
     case ExpressionKind::BoolTypeLiteral:
@@ -232,6 +234,9 @@ void Expression::PrintID(llvm::raw_ostream& out) const {
     case ExpressionKind::IdentifierExpression:
       out << cast<IdentifierExpression>(*this).name();
       break;
+    case ExpressionKind::DotSelfExpression:
+      out << ".Self";
+      break;
     case ExpressionKind::IntLiteral:
       out << cast<IntLiteral>(*this).value();
       break;