Parse support for where operator (#4275)

Includes support for the `impls`, `=`, and `==` requirement operators to
the right of a `where`, but `and` to allow multiple requirements is
still a TODO.

---------

Co-authored-by: Josh L <josh11b@users.noreply.github.com>
Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/parse/handle_expr.cpp

@@ -323,6 +323,15 @@ auto HandleExprLoop(Context& context) -> void {
         state.state = State::ExprLoopForShortCircuitOperatorAsOr;
         break;
 
+      // `where` also needs a virtual parse tree node, and parses its right
+      // argument in a mode where it can handle requirement operators like
+      // `impls` and `=`.
+      case Lex::TokenKind::Where:
+        context.AddNode(NodeKind::WhereOperand, state.token, state.has_error);
+        context.PushState(state, State::WhereFinish);
+        context.PushState(State::RequirementBegin);
+        return;
+
       default:
         state.state = State::ExprLoopForInfixOperator;
         break;