[Lit] Pass through DFLTCC from the surrounding environment

[iii]

Summary

Many distros ship zlib with the IBM Z deflate hardware acceleration
patch [1]. Sometimes it's desirable to disable the acceleration, for
example, for reproducibility. This can be done by exporting DFLTCC=0.
llvm-lit clears this environment variable, which causes
compress-debug-sections-zlib.test fail on z15 and later machines. Add
DFLTCC to the list of variables to keep.

[1] https://github.com/madler/zlib/pull/410

llvm/utils/lit/lit/TestingConfig.py

def fromdefaults(litConfig):
             'FILECHECK_OPTS',
             'VCINSTALLDIR',
             'VCToolsinstallDir',
             'VSINSTALLDIR',
             'WindowsSdkDir',
             'WindowsSDKLibVersion',
             'SOURCE_DATE_EPOCH',
             'GTEST_FILTER',
+            'DFLTCC',
         ]
 
         if sys.platform == 'win32':
             pass_vars += [
                 'COMSPEC',
                 'INCLUDE',
                 'LIB',
                 'PATHEXT',