Msvc has trouble defining a struct/class and defining a constexpr symbol in the same declarator. It reports the following error:
basic-parsers.h(809): error C2131: expression did not evaluate to a constant basic-parsers.h(809): note: failure was caused by call of undefined function or one not declared 'constexpr' basic-parsers.h(809): note: see usage of 'Fortran::parser::OkParser::OkParser'
Fix the msvc compilation by splitting the two definitions into two separate declarators.
This patch is part of the series to make flang compilable with MS Visual Studio.