conditionally ignore fatal diagnostic in the SilentEmitter

This change is primarily meant to allow rustfmt to ignore all
diagnostics when using the `SilentEmitter`. Back in PR 121301 the
`SilentEmitter` was shared between rustc and rustfmt. This changed
rustfmt's behavior from ignoring all diagnostic to emitting fatal
diagnostics.

These changes allow rustfmt to maintain it's previous behaviour when
using the SilentEmitter, while allowing rustc code to still emit fatal
diagnostics.

compiler/rustc_session/src/parse.rs

@@ -269,7 +269,11 @@ impl ParseSess {
         }
     }
 
-    pub fn with_silent_emitter(locale_resources: Vec<&'static str>, fatal_note: String) -> Self {
+    pub fn with_silent_emitter(
+        locale_resources: Vec<&'static str>,
+        fatal_note: String,
+        emit_fatal_diagnostic: bool,
+    ) -> Self {
         let fallback_bundle = fallback_fluent_bundle(locale_resources, false);
         let sm = Lrc::new(SourceMap::new(FilePathMapping::empty()));
         let emitter = Box::new(HumanEmitter::new(
@@ -281,6 +285,7 @@ impl ParseSess {
             fallback_bundle,
             fatal_dcx,
             fatal_note: Some(fatal_note),
+            emit_fatal_diagnostic,
         }))
         .disable_warnings();
         ParseSess::with_dcx(dcx, sm)