Implement lowering of if-let guards to MIR

Author: LeSeulArtichaut

clippy_lints/src/utils/author.rs

@@ -730,6 +730,7 @@ fn desugaring_name(des: hir::MatchSource) -> String {
             "MatchSource::IfLetDesugar {{ contains_else_clause: {} }}",
             contains_else_clause
         ),
+        hir::MatchSource::IfLetGuardDesugar => "MatchSource::IfLetGuardDesugar".to_string(),
         hir::MatchSource::IfDesugar { contains_else_clause } => format!(
             "MatchSource::IfDesugar {{ contains_else_clause: {} }}",
             contains_else_clause