Rollup merge of #69038 - yaahc:backtrace-debug, r=dtolnay
Rollup merge of #69038 - yaahc:backtrace-debug, r=dtolnay

Add initial debug fmt for Backtrace

Fixes the first point in https://github.com/rust-lang/rust/issues/65280

related to https://github.com/rust-lang/rust/issues/53487

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust