[Fortran] Forward NO_STOP_MESSAGE to FCVS tests

[rovka]

Summary

By default, Flang outputs 'Fortran STOP' as part of the STOP statement,
and sometimes also warnings about signaling exceptions, which are
platform-dependent. This output will cause test failures, but it can be
suppressed by setting the environment variable NO_STOP_MESSAGE=1. This
patch makes sure that llvm-lit forwards the NO_STOP_MESSAGE variable to
the tests if it is set in the environment.

[rovka]

This is an alternative to https://reviews.llvm.org/D126279
I personally prefer the other patch, for reasons which I've explained there.
If we go with this approach, I'll also send a patch to the docs to document the need for setting this environment variable.

[sscalpone]

This solution looks reasonable for stop messages. I presume it kicks in when running individual tests as well as the suite.

Perhaps we can change the default settings for flang not to announce the signaling exceptions? I am not sure if this change would be to the driver or the runtime. Perhaps someone else can weigh in?

Fortran/UnitTests/fcvs21_f95/lit.local.cfg

 config.traditional_output = True
 config.single_source = True
+
+# Flang uses NO_STOP_MESSAGE to control the output of the STOP statement. If
+# it is present in the environment, we should forward it to the tests, otherwise
+# they might choke on warnings about signaling INEXACT exceptions.
+if 'NO_STOP_MESSAGE' in os.environ:
+  config.environment['NO_STOP_MESSAGE'] = os.environ['NO_STOP_MESSAGE']