Fix duplicate lifetime parameter

[bb1yd]

Closes #123713

The previous code suggest adding a higher-ranked lifetime for<'a> for regular case and explicit lifetime 'a for associated type. But it didn't check whether the lifetime 'a is already occupied and it never suggest the user to use the existing lifetime.

I think that it might be better if we put the diagnostics code for error E0637 together with E0106 in the same function add_missing_lifetime_specifiers_label, since they share some same logic.

The current code behaves differently when there are lifetimes in scope:

• 0 lifetime available: same behaviour, suggest for<'a> (regular case) or 'a (associated type)
• 1 lifetime available: suggest using this lifetime or create a new higher-ranked lifetime with a fresh name (for regular case)
• multiple lifetime available: suggest using one of the lifetime or create a new higher-ranked lifetime with a fresh name (for regular case)

The help message of introducing HRTB changed to the same with E0106.

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