Detect errors caused by `async` block in 2015 edition

Fix rust-lang#67204.

Add some regression tests

Closes rust-lang#29988
Closes rust-lang#34979
Pick up two snippets that have been fixed from rust-lang#67945 (shouldn't be closed yet!)

…acrum

tests: Fix warnings in `rust_test_helpers.c`

MSVC is silly and doesn't recognize `assert` as diverging.

Grammar

I've split this into individual changes so that people can veto individually. I'm not attached to most of them.

`zeroes` vs. `zeros` is why I took the effort to run this through app.grammarly (which disappointingly didn't even notice).

…dows, r=Mark-Simulacrum

Fix bootstrap failing on win32

```powershell
python x.py -h # or really any x.py command
```
would fail with
```
info: Downloading and building bootstrap before processing --help
      command. See src/bootstrap/README.md for help with common
      commands.
Updating only changed submodules
Submodules updated in 0.15 seconds
Traceback (most recent call last):
  File "x.py", line 11, in <module>
    bootstrap.main()
  File "C:\Users\Joshua\Projects\forks\rust\src\bootstrap\bootstrap.py", line 960, in main
    bootstrap(help_triggered)
  File "C:\Users\Joshua\Projects\forks\rust\src\bootstrap\bootstrap.py", line 925, in bootstrap
    build.build = args.build or build.build_triple()
  File "C:\Users\Joshua\Projects\forks\rust\src\bootstrap\bootstrap.py", line 731, in build_triple
    return default_build_triple()
  File "C:\Users\Joshua\Projects\forks\rust\src\bootstrap\bootstrap.py", line 184, in default_build_triple
    ostype = require(["uname", "-s"], exit=required).decode(default_encoding)
AttributeError: 'NoneType' object has no attribute 'decode'
```

This PR defers the `decode` call until after we're sure `ostype` and `cputype` are not `None`, as they would be on Windows since `uname` doesn't exist