stabilize std::path::Path::file_prefix

[mbhall88]

This PR proposes to stabilize std::path::Path::file_prefix.

I believe the first step is actually starting an FCP, but it was mentioned that the easiest way to achieve this was to open this stabilization PR first.

[rustbot]

r? @Amanieu

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[rustbot]

This PR modifies tests/run-make/. If this PR is trying to port a Makefile
run-make test to use rmake.rs, please update the
run-make port tracking issue
so we can track our progress. You can either modify the tracking issue
directly, or you can comment on the tracking issue and link this PR.

cc @jieyouxu

[mbhall88]

@rustbot modify labels: +T-libs-api

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[slanterns]

You should ask T-libs-api for FCP and merge, not me :)

[tgross35]

Note that I suggested creating this PR as a way to get it on the radar, but I am of the opinion that the iterator API proposed at #86319 (comment) may be a preferable alternative. They are not mutually exclusive, however.