[libc++abi] Allow use just compiled clang++ for tests

[rengolin]

Summary

Currently, the tests assume the system compiler is the one we want
to test, but if we build libcxxabi together with LLVM+Clang, it'll
get the wrong compiler.

This patch allows us to test if we have clang++ in our /bin directory,
and if so, use it.

[rengolin]

Ping? This seems like a no-brainer.

[jroelofs]

LGTM

[rengolin]

Thanks! r243752.

test/lit.cfg

if obj_root is None:
     if obj_root is None:
         import tempfile
         obj_root = tempfile.mkdtemp(prefix='libcxxabi-testsuite-')
         lit_config.warning('Creating temporary directory for object root: %s' %
                            obj_root)
 
 config.test_exec_root = os.path.join(obj_root, 'test')
 
+# Check if we have produced a Clang with this build (in-tree)
+this_clang = os.path.join(obj_root, '../../bin/clang++')
+if os.path.exists(this_clang):
+    config.cxx_under_test = this_clang
+
 cfg_variant = getattr(config, 'configuration_variant', 'libcxxabi')
 if cfg_variant:
     print 'Using configuration variant: %s' % cfg_variant
 
 # Load the Configuration class from the module name <cfg_variant>.test.config.
 config_module_name = '.'.join([cfg_variant, 'test', 'config'])
 config_module = __import__(config_module_name, fromlist=['Configuration'])
 
 configuration = config_module.Configuration(lit_config, config)
 configuration.configure()
 configuration.print_config_info()
 config.test_format = configuration.get_test_format()