Bug 1329306 - Also clobber the CMakeFiles directory; r=mystor

This directory can include variables for the toolchain path names,
for example, which are different from run to run on Windows TaskCluster
workers.

build/build-clang/build-clang.py

@@ -205,6 +205,8 @@ def build_one_stage(cc, cxx, ld, ar, ranlib,
     # previous CMake run is cleared before running it again.
     if os.path.exists(build_dir + "/CMakeCache.txt"):
         os.remove(build_dir + "/CMakeCache.txt")
+    if os.path.exists(build_dir + "/CMakeFiles"):
+        shutil.rmtree(build_dir + "/CMakeFiles")
 
     # cmake doesn't deal well with backslashes in paths.
     def slashify_path(path):