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 https://github.com/rust-lang/rust/commit/a678e3191197f145451c97c6cc884e15cae38186; 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:
https://github.com/rust-lang/rust/blob/edebf77e0090195bf80c0d8cda821e1bf9d03053/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"`
```

1 files changed, 4 insertions, 1 deletions

diff --git a/compiler/rustc_interface/src/interface.rs b/compiler/rustc_interface/src/interface.rs
index 0861bd290df..81585f8f4e4 100644
--- a/compiler/rustc_interface/src/interface.rs
+++ b/compiler/rustc_interface/src/interface.rs
@@ -75,7 +75,10 @@ pub fn parse_cfgspecs(cfgspecs: Vec<String>) -> FxHashSet<(String, Option<String
         let cfg = cfgspecs
             .into_iter()
             .map(|s| {
-                let sess = ParseSess::with_silent_emitter();
+                let sess = ParseSess::with_silent_emitter(Some(format!(
+                    "this error occurred on the command line: `--cfg={}`",
+                    s
+                )));
                 let filename = FileName::cfg_spec_source_code(&s);
                 let mut parser = new_parser_from_source_str(&sess, filename, s.to_string());