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@aa91871539ca518e81af58485df895f8db30496a 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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• [ ] Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

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[rustbot]

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cc @rust-lang/compiler

[apiraino]

@rustbot second

it was agreed how to proceed (comment)

[rustbot]

The final comment period is now complete, this major change is now accepted. :tada:

As the automated representative, I want to express gratitude to the author for their work and everyone else who contributed to this major change proposal.

If you think this major change shouldn't have been accepted, feel free to remove the major-change-accepted label and reopen this issue.

[!NOTE] Further progress is now tracked over at rust-lang/rust#149240.