Cleanup to librustc::session and related code

[Mark-Simulacrum]

No functional changes, just some cleanup.

This also creates the rustc_fs_util crate, but I can remove that change if desired. It felt a little odd to force crates to depend on librustc for some fs utilities; and also seemed good to generally keep the size of librustc lower (for compile times); fs_util will compile in parallel with essentially the first crate since it has no dependencies beyond std.

[rust-highfive]

r? @pnkfelix

(rust_highfive has picked a reviewer for you, use r? to override)

[pnkfelix]

@bors r+

[bors]

📌 Commit 49c6e33ee236b6ebbfcda57b4fc103bd235a1351 has been approved by pnkfelix

[bors]

🔒 Merge conflict

This pull request and the master branch diverged in a way that cannot be automatically merged. Please rebase on top of the latest master branch, and let the reviewer approve again.

How do I rebase?

Assuming self is your fork and upstream is this repository, you can resolve the conflict following these steps:

1. git checkout data-structures (switch to your branch)
2. git fetch upstream master (retrieve the latest master)
3. git rebase upstream/master -p (rebase on top of it)
4. Follow the on-screen instruction to resolve conflicts (check git status if you got lost).
5. git push self data-structures --force-with-lease (update this PR)

You may also read Git Rebasing to Resolve Conflicts by Drew Blessing for a short tutorial.

Please avoid the "Resolve conflicts" button on GitHub. It uses git merge instead of git rebase which makes the PR commit history more difficult to read.

Sometimes step 4 will complete without asking for resolution. This is usually due to difference between how Cargo.lock conflict is handled during merge and rebase. This is normal, and you should still perform step 5 to update this PR.

Error message

warning: Cannot merge binary files: src/Cargo.lock (HEAD vs. heads/homu-tmp)
Auto-merging src/librustc_metadata/schema.rs
Auto-merging src/librustc_metadata/encoder.rs
Auto-merging src/librustc_metadata/decoder.rs
Auto-merging src/librustc_metadata/cstore_impl.rs
Auto-merging src/librustc_incremental/lib.rs
Auto-merging src/librustc_fs_util/lib.rs
Auto-merging src/librustc_data_structures/svh.rs
Auto-merging src/librustc_data_structures/lib.rs
Auto-merging src/librustc_data_structures/fingerprint.rs
Auto-merging src/librustc_codegen_llvm/lib.rs
Auto-merging src/librustc_codegen_llvm/back/write.rs
Auto-merging src/librustc_codegen_llvm/Cargo.toml
CONFLICT (content): Merge conflict in src/librustc_codegen_llvm/Cargo.toml
Auto-merging src/librustc/ty/query/mod.rs
Auto-merging src/librustc/lib.rs
Auto-merging src/librustc/hir/mod.rs
Auto-merging src/librustc/hir/map/mod.rs
Auto-merging src/Cargo.lock
CONFLICT (content): Merge conflict in src/Cargo.lock
Automatic merge failed; fix conflicts and then commit the result.

[Mark-Simulacrum]

@bors r=pnkfelix

[bors]

📌 Commit b3510b08c559cbccb740e61030e8a97b7dd292a2 has been approved by pnkfelix

[Mark-Simulacrum]

Dropped the last commit to try and avoid conflicts

@bors r=pnkfelix

[bors]

📌 Commit 488726c094a645388752b0d09971ecb8743ce9dd has been approved by pnkfelix

[Mark-Simulacrum]

@bors r=pnkfelix

[bors]

📌 Commit a8e19be has been approved by pnkfelix

[bors]

⌛ Testing commit a8e19be with merge db1acaa...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: pnkfelix
Pushing db1acaa to master...