Remove build directory for each build on clang-x86_64-debian-fast.

git-svn-id: https://llvm.org/svn/llvm-project/zorg/trunk@374738 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/buildbot/osuosl/master/config/builders.py b/buildbot/osuosl/master/config/builders.py
index c72e26a..0c9829d 100644
--- a/buildbot/osuosl/master/config/builders.py
+++ b/buildbot/osuosl/master/config/builders.py
@@ -97,6 +97,7 @@
          'factory': UnifiedTreeBuilder.getCmakeWithNinjaBuildFactory(
                         llvm_srcdir="llvm.src",
                         obj_dir="llvm.obj",
+                        clean=True,
                         depends_on_projects=['llvm','clang','clang-tools-extra','compiler-rt'],
                         extra_configure_args=[
                             "-DCOMPILER_RT_BUILD_BUILTINS:BOOL=OFF",