Support parse/check/lower for char (#5901)

toolchain/check/testdata/builtins/char/basics.carbon and
toolchain/lower/testdata/builtins/char.carbon are probably the most
interesting tests here. The parse tests is required because this adds a
new node kind, and we need coverage of it; but the attached info is
minor. There's a fair amount of test churn here because I'm adding the
Core.Char and Core.CharLiteral types as new singletons.

My intent here is that `CharId` is always a unicode code point, even
when the type is a `Char` and thus must be a single UTF-8 code unit
(single byte). This mainly means the stored value of a `CharValue` can
be printed internally without knowing the type.

---------

Co-authored-by: josh11b <15258583+josh11b@users.noreply.github.com>
Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/parse/handle_expr.cpp

@@ -89,6 +89,11 @@ auto HandleExprInPostfix(Context& context) -> void {
       context.PushState(state);
       break;
     }
+    case Lex::TokenKind::CharLiteral: {
+      context.AddLeafNode(NodeKind::CharLiteral, context.Consume());
+      context.PushState(state);
+      break;
+    }
     case Lex::TokenKind::IntLiteral: {
       context.AddLeafNode(NodeKind::IntLiteral, context.Consume());
       context.PushState(state);