[SystemZ][z/OS]: Manually create lit tmp_dir

[fanbo-meng]

Summary

Follow up of https://reviews.llvm.org/D118071. tempfile.mkdtemp() can still potentially include '_' character in the generated name, so we need to manually create lit temp directory name for z/OS to ensure it does not contain '_' character.

[fanbo-meng]

update comment

[abhina.sreeskantharajan]

LGTM

llvm/utils/lit/lit/main.py

 def execute_in_tmp_dir(run, lit_config):
     # Create a temp directory inside the normal temp directory so that we can
     # try to avoid temporary test file leaks. The user can avoid this behavior
     # by setting LIT_PRESERVES_TMP in the environment, so they can easily use
     # their own temp directory to monitor temporary file leaks or handle them at
     # the buildbot level.
     tmp_dir = None
     if 'LIT_PRESERVES_TMP' not in os.environ:
+        # z/OS linker does not support '_' in paths, so use '-' and letters.
+        if sys.platform == 'zos':
+            import random
+            import string
+            while True:
+                tmp_dir = '/tmp/lit-tmp-' + ''.join(random.choice(string.ascii_letters) for i in range(8))
+                if not os.path.exists(tmp_dir): break
+            os.makedirs(tmp_dir, mode=700)
+        else:
             import tempfile
-        # z/OS linker does not support '_' in paths, so use '-'.
-        tmp_dir = tempfile.mkdtemp(prefix='lit-tmp-')
+            tmp_dir = tempfile.mkdtemp(prefix='lit_tmp_')
         tmp_dir_envs = {k: tmp_dir for k in ['TMP', 'TMPDIR', 'TEMP', 'TEMPDIR']}
         os.environ.update(tmp_dir_envs)
         for cfg in {t.config for t in run.tests}:
             cfg.environment.update(tmp_dir_envs)
     try:
         run.execute()
     finally:
         if tmp_dir: