[flang] Use fundamental types in overloadings of Unparse.

Different systems map std::size_t, string::size_type, etc. to different
fundamental types. To ensure they are all covered, make the overloadings
of Unparse for integer types use only fundamental types.

This fixes compilations problems on Darwin and *BSD systems.

Original-commit: flang-compiler/f18@5576ed49a2ebe4bfae4a53772a6e0a0d98c18fa0
Reviewed-on: https://github.com/flang-compiler/f18/pull/205