[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

llvm-svn: 366700