Use list comprehension for filter

In python3, 'filter' returns an interable which doesn't support using
'len()'. This switches to a list comprehension, so that we can check the
length of 'enabled_pipeline'.

This is required to move build-script to Python3.

Author: jorng

utils/swift_build_support/swift_build_support/productpipeline_list_builder.py

@@ -141,7 +141,7 @@ def infer(self):
 
             # Filter out any of the pipelines that before inference were not
             # selected.
-            enabled_pipeline = filter(lambda x: x is not None, pipeline)
+            enabled_pipeline = [p for p in pipeline if p is not None]
 
             if self.args.verbose_build:
                 print("-- Build Graph Inference --")