[flang] Fix bug with intrinsic in type declaration stmt

When an instrinsic function is declared in a type declaration statement
we need to set the INTRINSIC attribute and (per 8.2(3)) ignore the
specified type.

To simplify the check, add IsIntrinsic utility to BaseVisitor.

Also, intrinsics and external procedures were getting assigned a size
and offset and they shouldn't be.

Differential Revision: https://reviews.llvm.org/D84702