This patch also moves `Regions` to a different module, since that type can stay
put when the FFI bindings move to another crate.

Not needed during tests.

ensure_version_or_cargo_install uses -Copt-level=0 for quicker installation.
However, the flag affects the tool's performance. For example, typos-cli with opt-level=0 takes 15 seconds for checking ./compiler, but the tool with default opt-level only takes less than 1 sec.
This commit enables the option only when the test tidy is run on CI.

cg_llvm: Remove inherent methods from several LLVM FFI types

This is mainly motivated by rust-lang#142897, which proposes to move the LLVM FFI bindings out of `rustc_codegen_llvm` and into `rustc_llvm`, which is arguably the more correct place for them from a linking perspective.

---

In order to perform that migration, all of the types used in FFI signatures also need to be moved. However, several of those types have inherent methods that convert from backend-independent types to LLVM FFI types.

Moving the inherent methods as-is would require adding a lot of otherwise-unnecessary dependencies to `rustc_llvm`. And we can't leave them behind as-is, because inherent methods can't be defined in another crate.

Therefore, this PR replaces several of those inherent methods with either extension trait methods or free functions.

Set opt-level flag for installing tool only on CI

## Context
ensure_version_or_cargo_install uses -Copt-level=0 for quicker installation. However, the flag affects the tool's performance. For example, typos-cli with opt-level=0 takes 15 seconds for checking /compiler on my local, but the tool with default opt-level only takes less than 1 sec.

It fixes rust-lang#147331

## Changes
This PR enables the option only when the test tidy is run on CI.
We've discussed that opt-level=1 should be enough, so I also change the level from 0 to 1.

… r=Kobzol

bootstrap: don't build book redirect pages during dry-run/test

Currently, `./x test bootstrap` does not automatically transitively checkout submodules needed to pass all involved test steps. Apparently one place where bootstrap's self-test can choke on locally is trying to build book redirect pages without the book submodules checked out.

This change is orthogonal to making bootstrap checking out required submodules for self-tests, and IMO is beneficial regardless since IMO we should not be building these redirect pages during test/dry-run _anyway_.

This was blocking me trying to rebless bootstrap self-tests for rust-lang#147372. cf. [#t-infra/bootstrap > Bootstrap self-tests @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Bootstrap.20self-tests/near/543157194).

r? `@Kobzol` (or bootstrap)

…f-test, r=Kobzol

bootstrap: relax `compiler-rt` root assertion

Not needed during tests. This would otherwise require `src/llvm-project` submodule to be initialized and `compiler-rt/` subdirectory to be present.

I need this and rust-lang#147374 to get bootstrap self-tests to pass with no git submodules checked out locally.

r? `@Kobzol` (or bootstrap)