Auto merge of #47098 - estebank:immutable-field, r=nagisa
Reword trying to operate in immutable fields
The previous message ("cannot assign/mutably borrow immutable field")
when trying to modify a field of an immutable binding gave the
(incorrect) impression that fields can be mutable independently of their
ADT's binding. Slightly reword the message to read "cannot
assign/mutably borrow field of immutable binding".
Re #35937.