c++: -fconcepts and __cpp_concepts
Since -fconcepts no longer implies -fconcepts-ts, we shouldn't advertise TS
support with __cpp_concepts=201507L. Also fix one case where -std=c++14
-fconcepts wasn't working (as found by range-v3 calendar). Fixing other
cases is not a priority, probably better to reject that flag combination if
there are further issues.
gcc/c-family/ChangeLog:
* c-cppbuiltin.cc (c_cpp_builtins): Adjust __cpp_concepts.
gcc/cp/ChangeLog:
* parser.cc (cp_parser_simple_type_specifier): Handle -std=c++14
-fconcepts.