[`bad_bit_mask`] Fix FP on proc macros (#13736)

`clap-3.1.6` was not compiling on the rustc-perf benchmarks because of
this lint. Also, the user could not allow the lint on the macro itself
because it was checking Exprs, not whatever the input of `bitflags!` is.

```
error: &-masking with zero
   --> src/build/arg_settings.rs:197:1
    |
            (#[allow] here)
197 | / bitflags! {
198 | |     struct Flags: u32 {
199 | |         const REQUIRED         ...
200 | |         const MULTIPLE_OCC     ...
...   |
226 | |     }
227 | | }
    | |_^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#bad_bit_mask
    = note: this error originates in the macro `__impl_bitflags` which comes from the expansion of the macro `bitflags` (in Nightly builds, run with -Z macro-backtrace for more info)
```

changelog:  [`bad_bit_mask`]: Fix FP on procedural macros

0 files changed, 0 insertions, 0 deletions