NLL Diagnostic Review 3: Diagnostic mentions lack of initialization instead of incorrect mutability

[davidtwco]

@pnkfelix's review comments:

ui/borrowck/reassignment_immutable_fields_overlapping.nll.stderr AST-borrowck complains about the local being immutable. NLL complains about lack of initialization. This is probably expected, but we should double-check.

[pnkfelix]

triage: P-medium. (We should look into this, but even if its wrong, I'm not going to push to a backport its hypothetical bug fix.)