Build a new incr comp session dir from scratch every time

[bjorn3]

Rather than copying the old incr comp dir and then modifying it. This saves a copy/hardlink for files that are modified. And it removes the need for accurate work product tracking to avoid accumulating cruft, which is non-trivial. We don't accurately track the pre-LTO bitcode files for ThinLTO for example.

Based on #159225
Part of rust-lang/compiler-team#908

[rust-log-analyzer]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

    Checking rustc_borrowck v0.0.0 (/checkout/compiler/rustc_borrowck)
warning: unused variable: `bitcode_path`
    --> compiler/rustc_codegen_ssa/src/back/write.rs:2269:9
     |
2269 |     let bitcode_path = in_incr_comp_dir_sess(tcx.incr_comp_session.unwrap(), &filename);
     |         ^^^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_bitcode_path`
     |
     = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default

[RUSTC-TIMING] rustc_codegen_ssa test:false 3.565
error: `rustc_codegen_ssa` (lib) generated 1 warning (run `cargo fix --lib -p rustc_codegen_ssa` to apply 1 suggestion)

[rust-bors]

☔ The latest upstream changes (presumably #159728) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.