Auto merge of #122102 - Urgau:optimize-symbol-integer, r=cjgillot

Optimize `Symbol::integer` by utilizing in-place formatting

This PR optimize `Symbol::integer` by utilizing `itoa` in-place formatting instead of going through a dynamically allocated `String` and the format machinery.

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For some context: I was profiling `rustc --check-cfg` with callgrind and due to the way we currently setup all the targets and we end-up calling `Symbol::integer` multiple times for all the targets. Using `itoa` reduced the number of instructions.

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Author: bors