Rollup merge of #89468 - FabianWolff:issue-89358, r=jackh726

Report fatal lexer errors in `--cfg` command line arguments

Fixes #89358. The erroneous behavior was apparently introduced by `@Mark-Simulacrum` in a678e31911; the idea is to silence individual parser errors and instead emit one catch-all error message after parsing. However, for the example in #89358, a fatal lexer error is created here:
edebf77e00/compiler/rustc_parse/src/lexer/mod.rs (L340-L349)

This fatal error aborts the compilation, and so the call to `new_parser_from_source_str()` never returns and the catch-all error message is never emitted. I have therefore changed the `SilentEmitter` to silence only non-fatal errors; with my changes, for the rustc invocation described in #89358:
```sh
rustc --cfg "abc\""
```
I get the following output:
```
error[E0765]: unterminated double quote string
  |
  = note: this error occurred on the command line: `--cfg=abc"`
```

compiler/rustc_errors/src/emitter.rs

@@ -15,7 +15,7 @@ use rustc_span::{MultiSpan, SourceFile, Span};
 use crate::snippet::{Annotation, AnnotationType, Line, MultilineAnnotation, Style, StyledString};
 use crate::styled_buffer::StyledBuffer;
 use crate::{
-    CodeSuggestion, Diagnostic, DiagnosticId, Level, SubDiagnostic, SubstitutionHighlight,
+    CodeSuggestion, Diagnostic, DiagnosticId, Handler, Level, SubDiagnostic, SubstitutionHighlight,
     SuggestionStyle,
 };
 
@@ -523,14 +523,27 @@ impl Emitter for EmitterWriter {
     }
 }
 
-/// An emitter that does nothing when emitting a diagnostic.
-pub struct SilentEmitter;
+/// An emitter that does nothing when emitting a non-fatal diagnostic.
+/// Fatal diagnostics are forwarded to `fatal_handler` to avoid silent
+/// failures of rustc, as witnessed e.g. in issue #89358.
+pub struct SilentEmitter {
+    pub fatal_handler: Handler,
+    pub fatal_note: Option<String>,
+}
 
 impl Emitter for SilentEmitter {
     fn source_map(&self) -> Option<&Lrc<SourceMap>> {
         None
     }
-    fn emit_diagnostic(&mut self, _: &Diagnostic) {}
+    fn emit_diagnostic(&mut self, d: &Diagnostic) {
+        if d.level == Level::Fatal {
+            let mut d = d.clone();
+            if let Some(ref note) = self.fatal_note {
+                d.note(note);
+            }
+            self.fatal_handler.emit_diagnostic(&d);
+        }
+    }
 }
 
 /// Maximum number of lines we will print for a multiline suggestion; arbitrary.