[flang] Consolidate and enhance pointer assignment checks

Consolidate aspects of pointer assignment & structure constructor pointer component
checking from Semantics/assignment.cpp and /expression.cpp into /pointer-assignment.cpp,
and add a warning about data targets that are not definable objects
but not hard errors.  Specifically, a structure component pointer component data
target is not allowed to be a USE-associated object in a pure context by a numbered
constraint, but the right-hand side data target of a pointer assignment statement
has no such constraint, and that's the new warning.

Differential Revision: https://reviews.llvm.org/D146581