Update clang-tidy docs to suggest a release build of clang

[email protected]

Change-Id: I19f39e02003e9d51a3c9eed0c17fc9208b958ab1
Reviewed-on: https://chromium-review.googlesource.com/786692
Commit-Queue: Chris Watkins <[email protected]>
Reviewed-by: Chris Watkins <[email protected]>
Cr-Commit-Position: refs/heads/master@{#519036}
diff --git a/docs/clang_tidy.md b/docs/clang_tidy.md
index fcf2184..cd5d1cc9 100644
--- a/docs/clang_tidy.md
+++ b/docs/clang_tidy.md
@@ -28,7 +28,7 @@
 
 Instead of building with `"Unix Makefiles"`, generate build files for Ninja with
 ```
-cmake -GNinja ../llvm
+cmake -GNinja -DCMAKE_BUILD_TYPE=Release ../llvm
 ```
 
 Then, instead of using `make`, use ninja to build the clang-tidy binary with