[PATCH] [libcxx] Refactor lit.cfg.

[Jon Roelofs]

================
Comment at: test/lit.cfg:226
@@ +225,3 @@
+    def configure_src_root(self):
+        self.src_root = self.lit_config.params.get('libcxx_src_root', None)
+        if self.src_root is None:
----------------
This pattern:

        self.src_root = self.lit_config.params.get('libcxx_src_root', None)
        if self.src_root is None:
            self.src_root = getattr(self.config, 'libcxx_src_root', None)
            if self.src_root is None:
                self.src_root = ...

happens a lot. Maybe it's worth abstracting it out?

http://reviews.llvm.org/D4952