[flang] Handle type-bound procedures with alternate returns

If you specify a type-bound procedure with an alternate return, there
will be no symbol associated with that dummy argument.  In such cases,
the compiler's list of dummy arguments will contain a nullptr.  In our
analysis of the PASS arguments of type-bound procedures, we were
assuming that all dummy arguments had non-null symbols associated with
them and were using that assumption to get the name of the dummy
argument.  This caused the compiler to try to dereference a nullptr.

I fixed this by explicitly checking for a nullptr and, in such cases, emitting
an error message.  I also added tests that contain type-bound procedures with
alternate returns in both legal and illegal constructs to ensure that semantic
analysis is working for them.

Differential Revision: https://reviews.llvm.org/D98430