[docs/clang-tidy] Mention script to build tools with correct revision

Mention tools/clang/scripts/build_clang_tools_extra.py, which is an
easy way to build clang-tidy and clang-apply-replacements with the
the correct LLVM revision. This is what allowed me to write
crrev.com/c/3646293

Bug: None
Change-Id: Ibd4e48312a19acfb974228d41657da5addeae166
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3644796
Reviewed-by: Hans Wennborg <hans@chromium.org>
Auto-Submit: Victor Vianna <victorvianna@google.com>
Commit-Queue: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/main@{#1004163}

docs/clang_tidy.md

@@ -208,8 +208,12 @@ your `$PATH`. However, the system packaged binaries might be several versions
 behind Chromium's toolchain, so not all flags are guaranteed to work. If this is
 a problem, consider building clang-tidy from the same revision the current
 toolchain is using, rather than filing a bug against the toolchain component.
-
-Running clang-tidy is (hopefully) simple.
+This can be done as follows:
+```
+tools/clang/scripts/build_clang_tools_extra.py \
+    --fetch out/Release clang-tidy clang-apply-replacements
+```
+Running clang-tidy is then (hopefully) simple.
 1.  Build chrome normally.
 ```
 ninja -C out/Release chrome