Fixed a problem removing temp files

[PeteSteinfeld]

Summary

Before making this change, whenever I ran "check-flang", I'd get an
error message like:

llvm-lit: /mnt/c/GitHub/f18/c751/flang/build/bin/../../../llvm/utils/lit/lit/main.py:252: warning: Failed to delete temp directory '/tmp/lit_tmp_gOKUIh'

With this change, there's no such message in the output, and the temp
directory is successfully removed.

Note that my working environment is on Windows 10 running Windows
Subsystem for Linux using the Ubuntu app. I'm running Python version
2.7.1.

Earlier versions of Python do not contain shutil. It may be that this
module was available on Windows systems later than other platforms.
Upgrading my version of Python made the problem go away

I don't believe that timing was a problem since inserting a long delay
didn't fix things.

So I added some text to the error message recommending that the user
upgrade their version of Python if they run into this problem.

[yln]

LGTM, thanks! Discussed in https://reviews.llvm.org/D79802

llvm/utils/lit/lit/main.py

if 'LIT_PRESERVES_TMP' not in os.environ:
                 })
     try:
         run.execute()
     finally:
         if tmp_dir:
             try:
                 import shutil
                 shutil.rmtree(tmp_dir)
-            except:
-                # FIXME: Re-try after timeout on Windows.
-                lit_config.warning("Failed to delete temp directory '%s'" % tmp_dir)
+            except Exception as e: 
+                lit_config.warning("Failed to delete temp directory '%s', try upgrading your version of Python to fix this" % tmp_dir)
 
 
 def print_histogram(tests):
     test_times = [(t.getFullName(), t.result.elapsed)
                   for t in tests if t.result.elapsed]
     if test_times:
         lit.util.printHistogram(test_times, title='Tests')