Ensure inferred let pattern types are well-formed

[s7tya]

Fixes #150040

This registers a well-formedness obligation for inferred let pattern types when the pattern contains ref bindings.

Previously, let PAT; without an explicit type annotation could infer a non-well-formed pattern input type, such as [str; 2] or (str, str). Some bindings inside the pattern can still have well-formed local types, for example ref x gives x: &str, so checking only the binding locals missed the enclosing array or tuple type. As a result, invalid unsized array/tuple pattern types were accepted and could later lead to layout ICEs.

The new check registers a WF obligation for the inferred declaration type after pattern checking.

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[mejrs]

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