lit googletest.py: Don't raise StopIteration to end generator function

[hans]

Summary

The intention here seems to be to end the generator function, but with modern Python, raising StopIteration causes a runtime error (https://www.python.org/dev/peps/pep-0479/).

[thakis]

Thanks!

llvm/utils/lit/lit/formats/googletest.py

def getGTestTests(self, path, litConfig, localConfig):
 
         try:
             output = subprocess.check_output(list_test_cmd,
                                              env=localConfig.environment)
         except subprocess.CalledProcessError as exc:
             litConfig.warning(
                 "unable to discover google-tests in %r: %s. Process output: %s"
                 % (path, sys.exc_info()[1], exc.output))
-            raise StopIteration
+            return
 
         nested_tests = []
         for ln in output.splitlines(False):  # Don't keep newlines.
             ln = lit.util.to_string(ln)
 
             if 'Running main() from gtest_main.cc' in ln:
                 # Upstream googletest prints this to stdout prior to running
                 # tests. LLVM removed that print statement in r61540, but we