[flang] Settle ambiguity between C795 and C721

C721 says that a type parameter value of '*' is permitted in the type-spec
for a named constant; C795 says that such type parameters are allowed
in type-specs only for a few kinds of things, not including named
constants.  The interpretation seems to depend on context, with C721
applying to intrinsic types (i.e., character) and C795 applying only
to derived types.

Differential Revision: https://reviews.llvm.org/D146586