[clang][lit] mkdir before mkstemp in is_filesystem_case_insensitive() (#131036)

In the CMake build test_exec_root already exists here, but not in the gn
build, which causes this to fail.

Author: aeubanks

clang/test/lit.cfg.py

@@ -261,6 +261,7 @@ def have_host_clang_repl_cuda():
 
 
 def is_filesystem_case_insensitive():
+    os.makedirs(config.test_exec_root, exist_ok=True)
     handle, path = tempfile.mkstemp(prefix="case-test", dir=config.test_exec_root)
     isInsensitive = os.path.exists(
         os.path.join(os.path.dirname(path), os.path.basename(path).upper())