This is currently a wrapper to `SymbolExportLevel` but it allows
later addition of extra information.

This has two benefits:
1. There is a clearer mental model of how bootstrap works. Steps correspond to paths on disk unless it's strictly impossible for them to do so (e.g. dist components).
2. Bootstrap has better checks for internal consistency. This caught several issues:
  - `src/sanitizers` doesn't exist; I changed it to just be a `sanitizers` alias.
  - `src/tools/lld` doesn't exist; I removed it, since `lld` alone already works.
  - `src/llvm` doesn't exist; removed it since `llvm` and `src/llvm-project` both work.
  - `src/lldb_batchmode.py` doesn't exist, it was moved to `src/etc`.
  - `install` was still using `src/librustc` instead of `compiler/rustc`.
  - None of the tools in `dist` / `install` allowed using `src/tools/X` to build them. This might be intentional - I can change them to aliases if you like.

`CRATE_DEF_ID` and `CrateNum::as_def_id` are almost always what we want.

The macro was a reimplementation of the function.

Generate synthetic object file to ensure all exported and used symbols participate in the linking

Fix rust-lang#50007 and rust-lang#47384

This is the synthetic object file approach that I described in rust-lang#95363 (comment), allowing all exported and used symbols to be linked while still allowing them to be GCed.

Related rust-lang#93791, rust-lang#95363

r? `@petrochenkov`
cc `@carbotaniuman`

…Simulacrum

Require all paths passed to `ShouldRun::paths` to exist on disk

This has two benefits:
1. There is a clearer mental model of how bootstrap works. Steps correspond to paths on disk unless it's strictly impossible for them to do so (e.g. dist components).
2. Bootstrap has better checks for internal consistency. This caught several issues:
  - `src/sanitizers` doesn't exist; I changed it to just be a `sanitizers` alias.
  - `src/tools/lld` doesn't exist; I removed it, since `lld` alone already works.
  - `src/llvm` doesn't exist; removed it since `llvm` and `src/llvm-project` both work.
  - `src/lldb_batchmode.py` doesn't exist, it was moved to `src/etc`.
  - `install` was still using `src/librustc` instead of `compiler/rustc`.
  - None of the tools in `dist` / `install` allowed using `src/tools/X` to build them. This might be intentional - I can change them to aliases if you like.

Builds on rust-lang#95901 and should not be merged before.

Update books

## nomicon

1 commits in 11f1165e8a2f5840467e748c8108dc53c948ee9a..c7d8467ca9158da58ef295ae65dbf00a308752d9
2022-03-19 16:02:00 -0400 to 2022-04-06 14:26:54 +0900
- Change "writers" to "readers" for Deref. (rust-lang/nomicon#346)

## reference

7 commits in c97d14fa6fed0baa9255432b8a93cb70614f80e3..b5f6c2362baf932db9440fbfcb509b309237ee85
2022-03-19 18:18:10 -0700 to 2022-04-10 19:19:51 -0700
- Fix typo: `?` should be inside `<sup>` tags (rust-lang/reference#1190)
- Update aarch64 to use neon as fp (rust-lang/reference#1184)
- Boolean literal expressions (rust-lang/reference#1189)
- Document that unary negation of a signed integer literal cannot cause an overflow error (rust-lang/reference#1188)
- Document compatibility between declarative and procedural macro tokens (rust-lang/reference#1169)
- Document native library modifier syntax and the `whole-archive` modifier specifically (rust-lang/reference#1170)
- Numeric literal expressions and literal suffixes (rust-lang/reference#1177)

## book

8 commits in ea90bbaf53ba64ef4e2da9ac2352b298aec6bec8..765318b844569a642ceef7bf1adab9639cbf6af3
2022-03-28 21:59:34 -0400 to 2022-04-12 21:14:47 -0400
- Propagate nostarch edits to src
- Propagate updated test example code to nostarch snapshot
- Edits to nostarch edits
- edits from nostarch
- Fix error message for the example code
- update ch13-02 to reflect changes in rust-lang/book#2797
- Update to 1.59
- Edits to chapter 2 after tech review

## rust-by-example

4 commits in ec954f35eedf592cd173b21c05a7f80a65b61d8a..c2a98d9fc5d29c481d42052fbeccfde15ed03116
2022-03-22 11:09:06 -0300 to 2022-04-08 06:44:18 -0300
- Code highlight a variable (rust-lang/rust-by-example#1530)
- Add a comment to note that warnings may not be shown in a browser in the Variable Bindings section (rust-lang/rust-by-example#1529)
- Make all new types have UpperCamelCase names in code example in the Aliasing section (rust-lang/rust-by-example#1528)
- Replace `C` with C/C++ (rust-lang/rust-by-example#1527)

## rustc-dev-guide

6 commits in 155126b1d2e2cb01ddb1d7ba9489b90d7cd173ad..eeb5a83c15b6ae60df3e4f19207376b22c6fbc4c
2022-03-22 14:34:21 +0100 to 2022-04-11 23:29:48 +0900
- method-lookup.md improvements (rust-lang/rustc-dev-guide#1296)
- Consolidate crates.io convention section (rust-lang/rustc-dev-guide#1326)
- Update examples with 1.61.0-nightly (latest version) (rust-lang/rustc-dev-guide#1330)
- r-a: Use `python3 x.py` instead of `./x.py` (rust-lang/rustc-dev-guide#1335)
- Update miri.md: correct a minor typo (rust-lang/rustc-dev-guide#1334)
- Add example how lints can be feature gated

…ochenkov

Stop using CRATE_DEF_INDEX outside of metadata encoding.

`CRATE_DEF_ID` and `CrateNum::as_def_id` are almost always what we want.  We should not manipulate raw `DefIndex` outside of metadata encoding.

…=GuillaumeGomez

Fix snapshot --bless not working anymore in htmldocck

I broke it in rust-lang#95933

r? ``@GuillaumeGomez``

…ackh726

Use revisions instead of nll compare mode for `/self/` ui tests

r? `@jackh726`

Replace u8to64_le macro with u64::from_le_bytes

The macro was a reimplementation of the function.