librustc: Forbid identifiers that shadow in the same pattern in let
bindings and function arguments.

Issue #14581.

To fix code that this breaks, give the pattern identifiers different names.

[breaking-change]
auto merge of #14801 : pcwalton/rust/name-shadowing-in-one-pattern, r=alexcrichton

bindings and function arguments.

Issue #14581.

To fix code that this breaks, give the pattern identifiers different names.

[breaking-change]

r? @brson