Make coverage_str compatible with Clang 10 for Bazel Central Registry's Presubmits

PiperOrigin-RevId: 690703622

centipede/distill.cc

diff --git a/centipede/distill.cc b/centipede/distill.cc
index 84e8c13..7e85205 100644
--- a/centipede/distill.cc
+++ b/centipede/distill.cc
@@ -289,7 +289,9 @@
 
   Stats GetStats() {
     absl::MutexLock lock{&mu_};
-    stats_.coverage_str = (std::stringstream{} << seen_features_).str();
+    std::stringstream ss;
+    ss << seen_features_;
+    stats_.coverage_str = std::move(ss).str();
     return stats_;
   }