[flang][runtime] Handle aliasing in Assign()

Detect and handle LHS/RHS aliasing when effecting intrinsic
assignments via the Assign() runtime function.

Also: don't apply special handling for allocatable LHS when calling
a user-defined type-bound ASSIGNMENT(=) generic procedure for a
polymorphic type, and refactor some code into utility functions to
make Assign() more comprehensible.

Differential Revision: https://reviews.llvm.org/D144026