Report fatal lexer errors in --cfg command line arguments

[FabianWolff]

Fixes #89358. The erroneous behavior was apparently introduced by @Mark-Simulacrum in a678e31; 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:

rust/compiler/rustc_parse/src/lexer/mod.rs

Lines 340 to 349 in edebf77

	rustc_lexer::LiteralKind::Str { terminated } => {
	if !terminated {
	self.sess.span_diagnostic.span_fatal_with_code(
	self.mk_sp(start, suffix_start),
	"unterminated double quote string",
	error_code!(E0765),
	)
	}
	(token::Str, Mode::Str, 1, 1) // " "
	}

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:

rustc --cfg "abc\""

I get the following output:

error[E0765]: unterminated double quote string
  |
  = note: this error occurred on the command line: `--cfg=abc"`

[rust-highfive]

r? @estebank

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[jackh726]

r? rust-lang/compiler

[jackh726]

😂

[jackh726]

@bors r+ rollup

[bors]

📌 Commit 041212f has been approved by jackh726