[front_end] Change TypeLabeler to return LabeledString.

The methods 'TypeLabeler.labelConstant` and `TypeLabeler.labelType`
are changed so that instead of returning a list of `Object` (which the
caller needs to `join()`), they return a new `LabeledString` class
whose `toString()` method takes care of calling `join()`.

This simplifies the code generation of CFE diagnostic
`.withArgumentsOld` methods, because it's no longer necessary to be
careful about calling `join()` after *all* inputs have been converted;
instead, the inputs can be converted in whatever order is convenient,
and the final `join()` happens implicitly when the `LabeledString`s
are converted to `String`s as part of string interpolation.

In follow-up CLs, I plan to rework the code generator for
`withArgumentsOld` so that it's easy to generate a parallel
`withArguments` function that takes named arguments. Starting from a
less complex code generator will make that rework easier.

Change-Id: I6a6a696415b214b7743f8e23192f2320aa499987
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/447821
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>