* c.opt: Change -std=c++1y and -std=gnu++1y to be aliases for
-std=c++14 and -std=gnu++14, rather than the reverse.
* c-opts.c (c_common_handle_option): Change OPT_std_c__1y and
OPT_std_gnu__1y to OPT_std_c__14 and OPT_std_gnu__14.
* c-common.h (cxx_dialect): Remove cxx1y.
From-SVN: r214414
