Add an error and adjust the terminology slightly about augmented redirecting constructors

[eernstg]

This PR introduces an error for the case where a redirection (this(...)) is augmented by another redirection, and similarly for a redirecting factory.

This follows the treatment of super-initializers, for which it is also an error to include one in an augmenting declaration if the augmented declaration already has an explicitly declared super-initializer.

Other than that, it clarifies the terminology of the augmentation feature specification slightly, such that we keep this(...) and this.x = 42 clearly separated.

'redirecting initializer' is changed to 'redirection' when talking about generative redirecting constructors. The term is derived from <redirection>, and the word redirection is used in other specification documents about this kind of term. This also helps avoiding the confusion about this(...) possibly being an initializer list element (like x = 42 or assert(b)).

I also added a few words of commentary right after the phrase 'factory redirection' because this phrase is new (the grammar rule actually doesn't have a separate name for the token sequence covered by this phrase). Finally, I deleted a TODO comment which is obsolete today (there are no special grammar rules about augmented).

[eernstg]

Commented on two changes.

[munificent]

Thanks for clarifying these weird corners!

[eernstg]

Thanks!

[eernstg]

@sgrekhov, this should unblock a couple of PRs.