Have lit preserve SOURCE_DATE_EPOCH

This environment variable has been standardized for reproducible builds. Setting
it can help to have reproducible tests too, so keep it as part of the testing
env when set.

See https://reproducible-builds.org/docs/source-date-epoch/

Differential Revision: https://reviews.llvm.org/D108332

(cherry picked from commit 46c947af7e)

llvm/utils/lit/lit/TestingConfig.py

@@ -28,7 +28,7 @@ class TestingConfig(object):
                      'TMPDIR', 'TMP', 'TEMP', 'TEMPDIR', 'AVRLIT_BOARD',
                      'AVRLIT_PORT', 'FILECHECK_OPTS', 'VCINSTALLDIR',
                      'VCToolsinstallDir', 'VSINSTALLDIR', 'WindowsSdkDir',
-                     'WindowsSDKLibVersion']
+                     'WindowsSDKLibVersion', 'SOURCE_DATE_EPOCH']
 
         if sys.platform == 'win32':
             pass_vars.append('INCLUDE')