Auto merge of #98051 - davidtwco:split-dwarf-stabilization, r=wesleywiser

session: stabilize split debuginfo on linux

Stabilize the `-Csplit-debuginfo` flag...

- ...on Linux for all values of the flag. Split DWARF has been implemented for a few months, hasn't had any bug reports and has had some promising benchmarking for incremental debug build performance.
- ..on other platforms for the default value. It doesn't make any sense that `-Csplit-debuginfo=packed` is unstable on Windows MSVC when that's the default behaviour, but keep the other values unstable.

Author: bors