[flang][msvc] Split class declaration and constexpr variable definition. NFC.

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 [[ http://lists.llvm.org/pipermail/flang-dev/2020-July/000448.html | make flang compilable with MS Visual Studio ]].

Reviewed By: DavidTruby, klausler

Differential Revision: https://reviews.llvm.org/D85937