This commit replaces the new error that was being emitted in NLL type
check with a call to `report_selection_error` so that the same trait
error as before this PR is emitted.
This commit implements RFC 2203, allowing constants in array repeat
expressions.
Firstly, the check that the array repeat expression implements `Copy` is
removed and re-implemented in `rustc_mir::borrow_check::nll::type_check`
by emitting an error when the MIR contains a `Operand::Move` and the
type does not implement `Copy`.
Next, the `qualify_consts` pass is modified to construct a
`Candidate::Repeat` when it would be correct to promote a array repeat
expression.
Finally, the `promote_consts` pass is modified to promote the candidates
previously identified.
