rustc: add support for split debuginfo (#4092)

This is #3168 by @durin42, with fixes for some of the pre-merge checks.

This PR adapts the original change to support path mapping when passing
`-Zsplit-dwarf-out-dir=<path>`. In order to do this, it adds to
`construct_arguments` the option to pass `(format_string, File)` as
elements in the list form of `rustc_flags`. This PR also adds tests for
the new functionality.

The new changes on top of the original PR were written with assistance
by Gemini.

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Co-authored-by: Augie Fackler <augie@google.com>