Disable the unused-mut warning when variable is prefixed by underscore

[pnkfelix]

See bstrie's comment on Issue 3083.

In short, the unused-mut warning added by pull request #5966 should work similarly to the unused variable detection: the warning should be disabled if you stick an underscore on the front of the identifier.

[catamorphism]

Bug triage. Doesn't look like a blocker to me.

[huonw]

Triage: fixed when #10518 lands.