updates .gitignore

* `.lh`: Local History is a helpful Code extension for caching changes,
  but it adds a lot of metadata to an in-tree directory that pollutes
  version control workspaces.
* `build`: this is a common name for build output, which also pollutes
  version control workspaces.

Author: Christopher Di Bella

.gitignore

@@ -1,8 +1,10 @@
+build/*
 docs
 out
 .cache
 .vscode/c_cpp_properties.json
 .vscode/settings.json
 .gclient_previous_sync_commits
 .gclient_entries
+.lh/*
 third_party/llvm