clang tools: Add --bootstrap to the documentation for update.py

This lets us build the clang tools with clang not gcc, which appears to
have bugs!

Also work around the gcc bug in one place we know it happens anyways,
just because.

R=dcheng
BUG=580745

Review URL: https://codereview.chromium.org/1647733002

Cr-Commit-Position: refs/heads/master@{#371933}

docs/clang_tool_refactoring.md

@@ -97,7 +97,7 @@ doesn't work well for Chromium:
 Synopsis:
 
 ```shell
-tools/clang/scripts/update.py --force-local-build --without-android \
+tools/clang/scripts/update.py --bootstrap --force-local-build --without-android \
   --tools blink_gc_plugin plugins rewrite_to_chrome_style
 ```
 
@@ -109,6 +109,9 @@ subdirectories in
 [//tools/clang](https://chromium.googlesource.com/chromium/src/+/master/tools/clang).
 Generally, `--tools` should always include `blink_gc_plugin` and `plugins`: otherwise, Chromium won't build.
 
+It is important to use --bootstrap as there appear to be [bugs](https://crbug.com/580745)
+in the clang library this script produces if you build it with gcc, which is the default.
+
 ## Running
 First, build all chromium targets to avoid failures due to missing dependecies
 that are generated as part of the build: