borrowck: clearer error message for mutability violation

[birkenfeld]

Because the X in "cannot assign to X" can be quite involved, it is easy for the user to assume that
cannot assign to captured outer variable in anFnMutclosure
refers to a specialty of FnMut closures. This change should make it clearer that the "immutable" is the key point here.

Note: I haven't checked in what other cases this message is emitted. It didn't seem to be covered by any existing borrowck compile-fail checks. But since the variant is called MutabilityViolation it seemed safe to add the "immutable".

Fixes: #31871

[rust-highfive]

r? @pnkfelix

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[birkenfeld]

Ok, as seen in the test failures, this isn't as easy:

cannot assign to immutable immutable captured outer variable in an `FnOnce` closure `x`