Drop timestamp in generated source code (llvm#146822)

Fixes llvm#72206

This helps reproducible builds of libomp.so -- 
probably because LLVM's LTO computed a hash of inputs to generate its
symbol names.

note: if it is desired to keep the timestamp, we could instead use
[`SOURCE_DATE_EPOCH`](https://reproducible-builds.org/docs/source-date-epoch/)
to make it deterministic.

This PR was done while working on [reproducible builds for
openSUSE](https://en.opensuse.org/openSUSE:Reproducible_Builds).

Author: bmwiedemann

openmp/runtime/tools/message-converter.py

@@ -11,7 +11,6 @@
 #
 
 import argparse
-import datetime
 import os
 import platform
 import re
@@ -188,11 +187,10 @@ def insert_header(f, data, commentChar="//"):
     f.write(
         "{0} Do not edit this file! {0}\n"
         "{0} The file was generated from"
-        " {1} by {2} on {3}. {0}\n\n".format(
+        " {1} by {2}. {0}\n\n".format(
             commentChar,
             os.path.basename(data.filename),
             os.path.basename(__file__),
-            datetime.datetime.now().ctime(),
         )
     )