An attribute list can not appear here

[tonghao.yuan]

clang complains about the An attribute list can not appear here:

cxxopts/include/cxxopts.hpp

Line 1252 in e976f96

 CXXOPTS_NODISCARD 

move the CXXOPTS_NODISCARD before inline can make clang quiet

[Ilya Popov]

Same problem as #353

[jarro2783]

Duplicate of #353