[llvm] [lit/tests] Replace 'env -u' with more portable construct

[mgorny]

Summary

Set environment variables to empty values rather than attempting
to unset them via 'env -u', in order to fix NetBSD test regression
caused by r366980. POSIX does not guarantee that env(1) supports '-u'
option, and indeed NetBSD env(1) does not support it.

[jdenny]

LGTM. Thanks again.

[stella.stamenova]

This actually breaks the lit tests on Windows when the path to Python has spaces. For example:

Command Output (stdout):
--
$ ":" "RUN: at line 3"
note: command had no output on stdout or stderr
$ "not" "env" "FILECHECK_OPTS=" "FILECHECK_DUMP_INPUT_ON_FAILURE=" "C:\Program Files (x86)\Microsoft Visual Studio\Shared\Python36_64\python.exe" "E:/_work/18/s/llvm\utils\lit\lit.py" "-j" "1" "-v" "E:/_work/18/b/utils/lit/tests\Inputs/shtest-output-printing"
# command stderr:
C:\Program: can't open file 'Files': [Errno 2] No such file or directory

[mgorny]

Are you talking about this *specific* change or the original change that added env usage?

[stella.stamenova]

It could be either actually. I'll run the tests with just the first one and see if it reproduces. I *think* it's this one.

[stella.stamenova]

It is indeed https://reviews.llvm.org/D65156 which introduced the failure

llvm/utils/lit/tests/lit.cfg

 # suites in %{inputs}.  This test suite's results are then determined in part
 # by %{lit}'s textual output, which includes the output of FileCheck calls
 # within %{inputs}'s test suites.  Thus, %{lit} clears environment variables
 # that can affect FileCheck's output.
 config.substitutions.append(('%{inputs}', os.path.join(
     config.test_source_root, 'Inputs')))
 config.substitutions.append(('%{lit}',
     "{env} %{{python}} {lit}".format(
-        env="env -u FILECHECK_OPTS -u FILECHECK_DUMP_INPUT_ON_FAILURE",
+        env="env FILECHECK_OPTS= FILECHECK_DUMP_INPUT_ON_FAILURE=",
         lit=os.path.join(lit_path, 'lit.py'))))
 config.substitutions.append(('%{python}', '"%s"' % (sys.executable)))
 
 # Enable coverage.py reporting, assuming the coverage module has been installed
 # and sitecustomize.py in the virtualenv has been modified appropriately.
 if lit_config.params.get('check-coverage', None):
     config.environment['COVERAGE_PROCESS_START'] = os.path.join(
         os.path.dirname(__file__), ".coveragerc")