[lit] Passthrough CLANG_MODULE_CACHE_PATH env var

This environment variable can be set to control module
caching. It disables caching by setting the variable
empty. As such, it needs to be handled differently
from other environment variables here which are
assumed to not be empty.

llvm/utils/lit/lit/TestingConfig.py

@@ -77,6 +77,14 @@ def fromdefaults(litConfig):
             if val:
                 environment[var] = val
 
+        pass_vars_emptyable = [
+            'CLANG_MODULE_CACHE_PATH',
+        ]
+
+        for var in pass_vars_emptyable:
+            if var in os.environ:
+                environment[var] = os.environ[var]
+
         # Set the default available features based on the LitConfig.
         available_features = []
         if litConfig.useValgrind: