Fix a move-checker diagnostic message (textual change) #82862
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The move-checker was assuming that any non-Copyable variable in a box must be
captured by a closure. The underlying problem is that the move-checker relies on
the best-effort AllocBoxToStack optimization to be perfect. But when
non-Escapable values depend on the variable, it remains boxed. That's good for
lifetime diagnostics but caused an incorrect move-checker diagnostic.
Fixes rdar://154519148 (Returning non-copyable type after accessing borrowed
field emits incorrect error about escaped closure capturing the noncopyable)