7047: Add force_show_panics flag for proc-macro bridge r=jonas-schievink a=edwin0cheng
https://github.com/rust-lang/rust/pull/75082 and https://github.com/rust-lang/rust/pull/76292 added a new flag in `proc_macro::Bridge` such that the ABI was changed. These ABI changing are the reason of some weird panics which caused #6880 and maybe related to the panic mentioned in #6820.
These changes are landed on rust stable 1.48 so I think it is okay to apply it now.
fixes#6880
r @jonas-schievink
Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>
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