[runtime/tools] Fix clang-tidy invocation

c++17 should be passed to -std=... and not to -x option.

Change-Id: I13e21d0ec98bcebe2ca537903e1bd399e287852a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/198401
Reviewed-by: Clement Skau <cskau@google.com>
Commit-Queue: Vyacheslav Egorov <vegorov@google.com>
diff --git a/runtime/tools/run_clang_tidy.dart b/runtime/tools/run_clang_tidy.dart
index d85f7c3..6e35eba 100644
--- a/runtime/tools/run_clang_tidy.dart
+++ b/runtime/tools/run_clang_tidy.dart
@@ -22,8 +22,9 @@
     '-DDEBUG',
     '-DTARGET_OS_LINUX',
     '-DTESTING',
+    '-std=c++17',
     '-x',
-    'c++17',
+    'c++',
   ];
   return flags;
 }