[flang] Make Fortran form of intrinsic types uppercase

`DynamicType::AsFortran` was using mixed case for intrinic type names.
Make it upper case for consistency with TYPE(...) and CHARACTER when a
length is present and other error messages.

Original-commit: flang-compiler/f18@e16909d67f16425b267511e5fe33f2ff72f69ee9
Reviewed-on: https://github.com/flang-compiler/f18/pull/928
Tree-same-pre-rewrite: false