[libc++] Do not infer support for C++17 in GCC < 7

libc++'s lit configuration infers the C++ language dialect when it is
not provided by checking which -std= flags that a compiler supports.
GCC 5 and GCC 6 have a -std=c++17 flag, however, they do not have full
C++17 support. The lit configuration has hardcoded logic that removes
-std=c++1z as an option to test for GCC < 7, but not -std=c++17.

This leads to a bunch of failures when running libc++ tests with GCC 5
or GCC 6. This patch adds -std=c++17 to the list of flags that are
discarded for GCC < 7 by lit's language dialect inference.

Thanks to Bryce Adelstein Lelbach for the patch.

Differential Revision: https://reviews.llvm.org/D62874

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@366700 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/utils/libcxx/test/config.py b/utils/libcxx/test/config.py
index e263e9d..47b8e8a 100644
--- a/utils/libcxx/test/config.py
+++ b/utils/libcxx/test/config.py
@@ -524,6 +524,7 @@
                 maj_v = int(maj_v)
                 if maj_v < 7:
+                    possible_stds.remove('c++17')
                 # FIXME: How many C++14 tests actually fail under GCC 5 and 6?
                 # Should we XFAIL them individually instead?
                 if maj_v <= 6: