Merge `CompilerError::CompilationFailed` and `CompilerError::ICE`.

`CompilerError` has `CompilationFailed` and `ICE` variants, which seems
reasonable at first. But the way it identifies them is flawed:
- If compilation errors out, i.e. `RunCompiler::run` returns an `Err`,
  it uses `CompilationFailed`, which is reasonable.
- If compilation panics with `FatalError`, it catches the panic and uses
  `ICE`. This is sometimes right, because ICEs do cause `FatalError`
  panics, but sometimes wrong, because certain compiler errors also
  cause `FatalError` panics. (The compiler/rustdoc/clippy/whatever just
  catches the `FatalError` with `catch_with_exit_code` in `main`.)

In other words, certain non-ICE compilation failures get miscategorized
as ICEs. It's not possible to reliably distinguish the two cases, so
this commit merges them. It also renames the combined variant as just
`Failed`, to better match the existing `Interrupted` and `Skipped`
variants.

Here is an example of a non-ICE failure that causes a `FatalError`
panic, from `tests/ui/recursion_limit/issue-105700.rs`:
```
 #![recursion_limit="4"]
 #![invalid_attribute]
 #![invalid_attribute]
 #![invalid_attribute]
 #![invalid_attribute]
 #![invalid_attribute]
 //~^ERROR recursion limit reached while expanding

 fn main() {{}}
```

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