Log timestamps in the bootstrap scripts (#8113)

### Motivation:

This enables detailed analysis of timings of different build phases of
the bootstrap scripts.

### Modifications:

Log messages as produced by `Utilities/helpers.py` also output
timestamps in the ISO format.

### Result:

It's easier to analyze build phases timing.

Author: MaxDesiatov

Utilities/bootstrap

@@ -22,7 +22,7 @@ import platform
 import re
 import shutil
 import subprocess
-from helpers import note, error, symlink_force, mkdir_p, call, call_output
+from helpers import note, error, symlink_force, mkdir_p, call, call_output, log_timestamp
 
 g_macos_deployment_target = '12.0'
 
@@ -840,4 +840,6 @@ def get_swiftpm_flags(args):
     return build_flags
 
 if __name__ == '__main__':
+    log_timestamp("start")
     main()
+    log_timestamp("end")

Utilities/helpers.py

@@ -11,17 +11,23 @@
 ##
 ##===----------------------------------------------------------------------===##
 
+import datetime
 import subprocess
 import sys
 import os
 import errno
 
+def log_timestamp(marker):
+    print("--- %s: note: %s %s" % (os.path.basename(sys.argv[0]), marker, datetime.datetime.now().isoformat()))
+
 def note(message):
     print("--- %s: note: %s" % (os.path.basename(sys.argv[0]), message))
+    log_timestamp("timestamp")
     sys.stdout.flush()
 
 def error(message):
     print("--- %s: error: %s" % (os.path.basename(sys.argv[0]), message))
+    log_timestamp("timestamp")
     sys.stdout.flush()
     raise SystemExit(1)