Rollup merge of #103142 - fmease:fix-103052, r=oli-obk

Make diagnostic for unsatisfied `Termination` bounds more precise

Don't blindly emit a diagnostic claiming that “*`main` has an invalid return type*” if we encounter a type that should but doesn't implement `std::process::Termination` and isn't actually the return type of the program entry `main`.

Fixes #103052.

``@rustbot`` label A-diagnostics T-compiler T-libs
r? diagnostics

compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs

@@ -164,6 +164,10 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             flags.push((sym::from_desugaring, Some(format!("{:?}", k))));
         }
 
+        if let ObligationCauseCode::MainFunctionType = obligation.cause.code() {
+            flags.push((sym::cause, Some("MainFunctionType".to_string())));
+        }
+
         // Add all types without trimmed paths.
         ty::print::with_no_trimmed_paths!({
             let generics = self.tcx.generics_of(def_id);