Bug 1249858 - Do not generate unified C++ files during artifact builds; r=glandium

I can't think of a good reason why unified C++ files need to exist
during artifact builds. Let's not write them in this build config.

Before: 2517 total backend files; 2517 created; 0 updated; 0 unchanged; 0 deleted
After:  2001 total backend files; 2001 created; 0 updated; 0 unchanged; 0 deleted

No measureable change in performance on Linux. But on Windows where file
creation is in the ~1ms range, this will surely result in a speedup of
several hundred milliseconds.

We are still generating some .cpp files during artifact builds. This will be
addressed in subsequent commits.

MozReview-Commit-ID: Lqx36YE8qZZ

--HG--
extra : rebase_source : 8af5a9915e78af7aee2aa335552689fe33975400

python/mozbuild/mozbuild/backend/common.py

@@ -254,6 +254,10 @@ class CommonBackend(BuildBackend):
             self._ipdl_sources.add(mozpath.join(obj.srcdir, obj.basename))
 
         elif isinstance(obj, UnifiedSources):
+            # Unified sources aren't relevant to artifact builds.
+            if self.environment.is_artifact_build:
+                return True
+
             if obj.have_unified_mapping:
                 self._write_unified_files(obj.unified_source_mapping, obj.objdir)
             if hasattr(self, '_process_unified_sources'):