[flang] Enforce C1529 as a warning, C919 as an error

Constraint C1529 requires that the base object of a type-bound procedure
reference be a scalar if the TBP has the NOPASS attribute.  Most
compilers do not enforce this constraint and it does not appear to
have any implementation justification, so emit portability warning.

On the other hand, we fail to enforce C919 for references to
procedure pointer components, whose base objects must of course
be scalars in order to avoid ambiguity and empty arrays, whether
NOPASS is present or not.

Differential Revision: https://reviews.llvm.org/D140148