[bad_bit_mask] Fix FP on proc macros

[blyxyas]

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

[rustbot]

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[Jarcho]

Thank you.