Rollup merge of #47328 - mbrubeck:fs_read, r=sfackler

Use the new fs_read_write functions in rustc internals

Uses `fs::read` and `fs::write` (added by #45837) where appropriate, to simplify code and dog-food these new APIs.  This also improves performance, when combined with #47324.

diff --cc src/librustc_back/target/mod.rs
Simple merge

diff --cc src/librustc_incremental/assert_dep_graph.rs
Simple merge

diff --cc src/librustc_incremental/persist/save.rs
Simple merge