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"`
```

0 files changed, 0 insertions, 0 deletions