Teach test suite about C++2a dialect flag.

This patch teaches the test suite configuration about the -std=c++2a
flag. And, since it's the newest dialect, change the test suite to
choose it, if possible, by default.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@317611 91177308-0d34-0410-b5e6-96231b3b80d8

utils/libcxx/test/config.py

@@ -517,7 +517,8 @@ class Configuration(object):
         std = self.get_lit_conf('std')
         if not std:
             # Choose the newest possible language dialect if none is given.
-            possible_stds = ['c++17', 'c++1z', 'c++14', 'c++11', 'c++03']
+            possible_stds = ['c++2a', 'c++17', 'c++1z', 'c++14', 'c++11',
+                             'c++03']
             if self.cxx.type == 'gcc':
                 maj_v, _, _ = self.cxx.version
                 maj_v = int(maj_v)
@@ -888,7 +889,7 @@ class Configuration(object):
         # Turn on warnings by default for Clang based compilers when C++ >= 11
         default_enable_warnings = self.cxx.type in ['clang', 'apple-clang'] \
             and len(self.config.available_features.intersection(
-                ['c++11', 'c++14', 'c++17'])) != 0
+                ['c++11', 'c++14', 'c++17', 'c++2a'])) != 0
         enable_warnings = self.get_lit_bool('enable_warnings',
                                             default_enable_warnings)
         self.cxx.useWarnings(enable_warnings)