Sync 'in class initialization of static const double' extension up with GCC,

and split it out of -Wgnu into its own warning flag.

 * In C++11, this is now a hard error (GCC has no extension here in C++11 mode).
   The error can be disabled with -Wno-static-float-init, and has a fixit to
   add 'constexpr'.

 * In C++98, this is still an ExtWarn, but is now controlled by
   -Wstatic-float-init as well as -Wgnu.

llvm-svn: 173414