Faulty code: Make OCUndeclaredVariableWarning a little less special

[privat]

This PR does some cleanup and reorganization.

In #13165 it was explicitly noted that OCASTSemanticAnalyzer had to deal with OCUndeclaredVariableWarning signaling in non-faulty-mode.
This is bad because:

1. the semantic analyzer should not have to signal exceptions, but only annotate the AST.
2. OCUndeclaredVariableWarning is signaled too early, and possibly before more important errors.

So this PR remove this responsibility on the semantic analyzer by making the Undeclared thing compatible with the RBNotice and CodeError infrastructure: it just annotates the AST; then possible OCUndeclaredVariableWarning are signaled along with possible CodeError.
However, it does it transparently for existing clients, so preserving the current "half-warning half-error" semantic. (See #13006 )

Here is a summary on the changes:

• new special notice OCUndeclaredVariableNotice
• OCASTSemanticAnalyzer does not check faulty, nor signal exceptions on undeclared variables
• on faulty check, this new class OCUndeclaredVariableNotice does not signal CodeError but OCUndeclaredVariableWarning
• OCUndeclaredVariableWarning is made compatible with the API of CodeError, so catcher that care can be agnostic.
  Currently, it cannot be a subclass of CodeError because doing so breaks clients and tests that catch (or expect not to catch) CodeError exceptions.

As a side effect, OCSemanticWarning that is no more used is removed.