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() {{}}
```

compiler/stable_mir/src/error.rs

@@ -15,10 +15,8 @@ macro_rules! error {
 /// An error type used to represent an error that has already been reported by the compiler.
 #[derive(Clone, Copy, PartialEq, Eq)]
 pub enum CompilerError<T> {
-    /// Internal compiler error (I.e.: Compiler crashed).
-    ICE,
-    /// Compilation failed.
-    CompilationFailed,
+    /// Compilation failed, either due to normal errors or ICE.
+    Failed,
     /// Compilation was interrupted.
     Interrupted(T),
     /// Compilation skipped. This happens when users invoke rustc to retrieve information such as
@@ -54,8 +52,7 @@ where
 {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         match self {
-            CompilerError::ICE => write!(f, "Internal Compiler Error"),
-            CompilerError::CompilationFailed => write!(f, "Compilation Failed"),
+            CompilerError::Failed => write!(f, "Compilation Failed"),
             CompilerError::Interrupted(reason) => write!(f, "Compilation Interrupted: {reason}"),
             CompilerError::Skipped => write!(f, "Compilation Skipped"),
         }
@@ -68,8 +65,7 @@ where
 {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         match self {
-            CompilerError::ICE => write!(f, "Internal Compiler Error"),
-            CompilerError::CompilationFailed => write!(f, "Compilation Failed"),
+            CompilerError::Failed => write!(f, "Compilation Failed"),
             CompilerError::Interrupted(reason) => write!(f, "Compilation Interrupted: {reason:?}"),
             CompilerError::Skipped => write!(f, "Compilation Skipped"),
         }