Have git ignore the third_party/llvm-bootstrap-install dir

third_party/llvm-bootstrap-install is sometimes created by
tools/clang/update.py and should not disturb git.

Change-Id: Id30390bc0e15e69661828f0124c08475539e3490
Reviewed-on: https://chromium-review.googlesource.com/966421Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#543687}

third_party/.gitignore

@@ -122,6 +122,7 @@
 /llvm
 /llvm-allocated-type
 /llvm-bootstrap
+/llvm-bootstrap-install
 /llvm-build
 /llvm-build-tools
 /lss