Don't complain re missing `mut` on attempt to partially initialize an uninitialized...

Don't complain re missing `mut` on attempt to partially initialize an uninitialized struct.

Under the semantics of #54986 (our short term plan), the partial
initialization itself will signal an error. We don't need to add noise
to the output by also complaining about `mut`. (In particular, the
user may well revise their code in a way that does not require `mut`.)