Add support for specifying split-debuginfo=unpacked output directory

[durin42]

Proposal

We've encountered users that depend on .dwo objects being directly-loadable by their debugger, rather than relying on the merged .dwp file at the end of the build of the binary or test. A complicating factor is that we're using bazel with remote execution exclusively, so we have to be able to list either exact filenames or directories of a single type before executing any tools[0]. Originally we were using a helper binary that wrapped rustc to move the .dwo files to a known-upfront directory, but this breaks debugging (experimentally lldb will print lots of warnings upon not finding the dwo file, gdb will just crash). As a result, we'd like a flag to control where the .dwo files get written - in my draft patch I used -Zsplit-dwarf-out-dir= but that's obviously open to revision.

I'm not sure if we should land this now: one immediate concern I have is that we haven't yet done FDO/PGO work in bazel, and that seems like it might have similar interactions with the compiler. Semi-related past work: rust-lang/rust@aa91871 where we added support for writing summary bitcode for distributed ThinLTO.

0: bazel calls this the analysis phase - as contrasted with the execution phase. It's a hard constraint in bazel's design that all artifacts it will collect be listed out by name before the execution phase starts.

Mentors or Reviewers

The actual patches are simple enough I can probably self-mentor? Not sure who should review.

Process

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[rustbot]

Important

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