[flang] Admit NULL() in generic procedure resolution cases

Semantics is rejecting valid programs with NULL() actual arguments
to generic interfaces, including user-defined operators.  Subclause
16.9.144(para 6) makes clear that NULL() can be a valid actual
argument to a generic interface so long as it does not produce
ambiguity.  This patch handles those cases, revises existing
tests, and adjust an error message about NULL() operands to
appear less like a blanket prohibition.

Differential Revision: https://reviews.llvm.org/D111850