`expand_pattern()` has two callsites. One of them needs arena
allocation, but the other does not.

This commit moves the arena allocation out of the function. This avoids
the allocation of many 4 KiB page arena chunks that only hold a single
small allocation. It reduces the number of bytes allocated by up to 2%
for various benchmarks, albeit without only a very small improvement in
runtime.

variables.

resolve_vars_with_obligations, as it now also resolves const variables.

…=alexcrichton

ci: move most of the prepare config into scripts

This PR moves most of the configuration from the CI yamls into bash scripts, driven by a small Python script (which understands and emulates the two `##vso[` commands we use).

There are two reasons why we'd want to do this:

* Being able to prepare the build environment locally by just running `src/ci/prepare.py` simplifies a lot setting up a local VM similar to CI (software pre-installed in the CI images won't be prepared, but it's a start anyway).
* When we'll switch to GitHub Actions we'll need to either duplicate code in multiple workflows or write a preprocessor. Having all the prepare steps in a single one is going to simplify the implementation of both options.

Along with the move I did a few changes to the actual scripts:

* Mirrored all the remaining external URLs we download (except chocolatey) to the `rust-lang-ci-mirrors` bucket, to increase reliability and reduce the chance of supply chain attacks. I didn't audit and mirror the CI scripts outside this PR though.
* Extracted CI-specific behavior (like issuing `##vso[` commands and detecting the host platform) into `shared.sh` and included it in most of the scripts. This way a switch to another CI provider will be less painful.

It's possible (and easier) to review this commit-by-commit.
r? @alexcrichton
cc @rust-lang/infra

Clean up `contains()` `insert()` chains on HashSet

They can be merged to a single `insert()` call, which may avoid double-hashing/lookup of the value.

…m-expand_pattern, r=varkor

Avoid unnecessary arena allocations in `expand_pattern()`.

`expand_pattern()` has two callsites. One of them needs arena
allocation, but the other does not.

This commit moves the arena allocation out of the function. This avoids
the allocation of many 4 KiB page arena chunks that only hold a single
small allocation. It reduces the number of bytes allocated by up to 2%
for various benchmarks, albeit without only a very small improvement in
runtime.

Changed `resolve_type_vars_with_obligations` to also resolve const inference variables

Fixes rust-lang#65380
r? @varkor

Remove unreachable unit tuple compare binop codegen

Closes rust-lang#63906
Fixes rust-lang#63551

This is based on rust-lang#63906 by @luigisHat, who had trouble with rebasing his PR.

trivial typo fix