libc++ testing: fix assertion around use_system_cxx_lib

Actually fix (hopefully) the assertions about `use_system_cxx_lib`,
the previous attempt failed because I misread the error.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@299056 91177308-0d34-0410-b5e6-96231b3b80d8

utils/libcxx/test/config.py

@@ -279,8 +279,8 @@ class Configuration(object):
             self.use_system_cxx_lib = True
         elif self.use_system_cxx_lib == 'false':
             self.use_system_cxx_lib = False
-        else:
-            assert os.path.isdir(str(self.use_system_cxx_lib))
+        else if self.use_system_cxx_lib:
+            assert os.path.isdir(self.use_system_cxx_lib)
         self.lit_config.note(
             "inferred use_system_cxx_lib as: %r" % self.use_system_cxx_lib)