Support for let declarations. (#3257)

A `let` declaration is represented by a `bind_name` node in SemIR:

```carbon-semir
  %b: i32 = bind_name "b", %a
```

Because `Check` encounters the pattern before it sees the value, we
first create the `bind_name` node with an unset value and don't add it
to the block. Then, once we've seen and converted the initializer, we
update the `bind_name` to have the value and add it to the current
block, after the initializer code.

toolchain/parse/handle_pattern.cpp

@@ -6,7 +6,7 @@
 
 namespace Carbon::Parse {
 
-// Handles PatternAs(DeducedParameter|FunctionParameter|Variable).
+// Handles PatternAs(DeducedParameter|FunctionParameter|Variable|Let).
 static auto HandlePattern(Context& context, Context::PatternKind pattern_kind)
     -> void {
   auto state = context.PopState();
@@ -36,6 +36,12 @@ static auto HandlePattern(Context& context, Context::PatternKind pattern_kind)
         context.emitter().Emit(*context.position(), ExpectedVariableName);
         break;
       }
+      case Context::PatternKind::Let: {
+        CARBON_DIAGNOSTIC(ExpectedLetBindingName, Error,
+                          "Expected pattern in `let` declaration.");
+        context.emitter().Emit(*context.position(), ExpectedLetBindingName);
+        break;
+      }
     }
     // Add a placeholder for the type.
     context.AddLeafNode(NodeKind::InvalidParse, *context.position(),
@@ -90,6 +96,10 @@ auto HandlePatternAsVariable(Context& context) -> void {
   HandlePattern(context, Context::PatternKind::Variable);
 }
 
+auto HandlePatternAsLet(Context& context) -> void {
+  HandlePattern(context, Context::PatternKind::Let);
+}
+
 // Handles PatternFinishAs(Generic|Regular).
 static auto HandlePatternFinish(Context& context, NodeKind node_kind) -> void {
   auto state = context.PopState();