Add an llvm work directory to .gitignore

Change-Id: I2a22b99caa64cc0d1c3b8d74425375b76b1dca67
Reviewed-on: https://chromium-review.googlesource.com/957737Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#542511}

third_party/.gitignore

@@ -123,6 +123,7 @@
 /llvm-allocated-type
 /llvm-bootstrap
 /llvm-build
+/llvm-build-tools
 /lss
 /material_design_icons/src
 /mesa/src