[libcxx] Always convert 'use_system_cxx_lib' to an absolute path

Otherwise, some tests would fail when a relative path was passed,
because they'd use the relative path from a different directory
than the current working directory.

llvm-svn: 348525

libcxx/utils/libcxx/test/config.py

@@ -307,6 +307,7 @@ class Configuration(object):
             self.use_system_cxx_lib = False
         elif self.use_system_cxx_lib:
             assert os.path.isdir(self.use_system_cxx_lib), "the specified use_system_cxx_lib parameter (%s) is not a valid directory" % self.use_system_cxx_lib
+            self.use_system_cxx_lib = os.path.abspath(self.use_system_cxx_lib)
         self.lit_config.note(
             "inferred use_system_cxx_lib as: %r" % self.use_system_cxx_lib)