rustc_incremental: avoid ICE when dumping dep graph fails

[cuishuang]

-Zdump-dep-graph used unwrap() when creating or writing the dependency graph output files. As a result, setting RUST_DEP_GRAPH to a path whose parent directory does not exist, or to another unwritable location, caused rustc to ICE.

This changes dump_graph to propagate filesystem errors and emit the existing structured file-writing diagnostic at the caller.

The run-make test now covers an invalid output directory and verifies that rustc reports a normal error without ICEing.

Tests:

• ./x test tests/run-make/dep-graph
• ./x fmt --check
• ./x test tidy

[rustbot]

r? @wesleywiser

rustbot has assigned @wesleywiser.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler, incremental
• compiler, incremental expanded to 74 candidates
• Random selection from 19 candidates

[rust-bors]

☔ The latest upstream changes (presumably #160102) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

[oli-obk]

We generally do not fix ICEs in internal feature gates or forever-unstable command line flags. It's usually something only used by ppl digging around in compiler internals anyway, so they also know how to debug it