enable range-loop-analysis warning

This is for
https://gn-review.googlesource.com/c/gn/+/13781/3#message-9ed3cea2037c3f884544388fb648486e6b28fe3c

I think this warning does not harm build.
https://clang.llvm.org/docs/DiagnosticsReference.html#wrange-loop-analysis

Change-Id: Ice3ef82ad578b1ca3b50643b0de8e9b2cb20c1aa
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/13800
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@google.com>
diff --git a/build/gen.py b/build/gen.py
index 6e915b8..e814b58 100755
--- a/build/gen.py
+++ b/build/gen.py
@@ -439,6 +439,7 @@
         '-Wall',
         '-Wextra',
         '-Wno-unused-parameter',
+        '-Wrange-loop-analysis',
         '-Wundef',
         '-std=c++17'
     ])
diff --git a/src/gn/analyzer.cc b/src/gn/analyzer.cc
index fd93879..e15c265 100644
--- a/src/gn/analyzer.cc
+++ b/src/gn/analyzer.cc
@@ -183,7 +183,7 @@
     }
   }
 
-  for (const auto& kv : dict->DictItems()) {
+  for (const auto kv : dict->DictItems()) {
     if (kv.first == kFilesKey || kv.first == kAdditonalCompileTargetsKey ||
         kv.first == kTestTargetsKey) {
       continue;