[compiler] Improve name hints for outlined functions

The previous PR added name hints for anonymous functions, but didn't handle the case of outlined functions. Here we do some cleanup around function `id` and name hints:
* Make `HIRFunction.id` a ValidatedIdentifierName, which involved some cleanup of the validation helpers
* Add `HIRFunction.nameHint: string` as a place to store the generated name hints which are not valid identifiers
* Update Codegen to always use the `id` as the actual function name, and only use nameHint as part of generating the object+property wrapper for debug purposes.

This ensures we don't conflate synthesized hints with real function names. Then, we also update OutlineFunctions to use the function name _or_ the nameHint as the input to generating a unique identifier. This isn't quite as nice as the object form since we lose our formatting, but it's a simple step that gives more context to the developer than `_temp` does.

Switching to output the object+property lookup form for outlined functions is a bit more involved, let's do that in a follow-up.