Fix -t clean to remove all outputs of multi-output edges

[philwo]

When cleaning a specific target with ninja -t clean <target>, DoCleanTarget only removed the named target file. For multi-output edges (e.g. build a.txt | b.txt: gen in.txt), the other outputs were left behind even though they are produced by the same command and cannot be rebuilt independently. The docs say "additional arguments are targets, which removes the given targets and recursively all files built for them", so I think this is a bug.

Fix by iterating over all edge outputs in DoCleanTarget, consistent with how CleanAll already handles multi-output edges.

[Copilot]

Pull request overview

Fixes ninja -t clean <target> so that cleaning a single target from a multi-output edge removes all outputs produced by that edge, aligning behavior with the documented “recursively all files built for them” semantics.

Changes:

• Update Cleaner::DoCleanTarget to remove every output in the producing edge (not only the named target).
• Add a unit test verifying multi-output edge cleanup behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/clean.cc	Ensures CleanTarget removes all outputs of the target’s generating edge before recursing.
src/clean_test.cc	Adds coverage for cleaning a target that is one of multiple outputs from the same edge.

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