Generate empty .clang-format-ignore before running tests (#136154)

Followup to #136022, this ensures formatting tests are run with an empty
`.clang-format-ignore` in their root directory, to prevent failures if
the file also exists higher in the tree.

Author: DKLoehr

clang/test/Format/lit.local.cfg

@@ -1,3 +1,4 @@
+import os
 import platform
 import lit.formats
 
@@ -27,3 +28,8 @@ config.suffixes = [
 # python implementation does, so use that for cross platform compatibility
 if platform.system() == "AIX":
     config.test_format = lit.formats.ShTest()
+
+# Create an empty .clang-format-ignore file so that tests don't get messed
+# up if one exists higher in the tree
+with open(".clang-format-ignore", 'w'):
+    pass