Auto merge of #115897 - eduardosm:check-fn-sig, r=compiler-errors

rustc_hir_analysis: add a helper to check function the signature mismatches

This function is now used to check `#[panic_handler]`, `start` lang item, `main`, `#[start]` and intrinsic functions.

The diagnosis produced are now closer to the ones produced by trait/impl method signature mismatch.

This is the first time I do anything with rustc_hir_analysis/rustc_hir_typeck, so comments and suggestions about things I did wrong or that could be improved will be appreciated.

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

@@ -2717,6 +2717,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             | ObligationCauseCode::IfExpressionWithNoElse
             | ObligationCauseCode::MainFunctionType
             | ObligationCauseCode::StartFunctionType
+            | ObligationCauseCode::LangFunctionType(_)
             | ObligationCauseCode::IntrinsicType
             | ObligationCauseCode::MethodReceiver
             | ObligationCauseCode::ReturnNoExpression