[flang] Fix associating entity when selector is an array, pointer or allocatable

In SELECT TYPE, within the block following TYPE IS, the associating entity is not polymorphic.
It has the type named in the type guard and other properties taken from the
selector. Within the block following a CLASS IS type guard statement, the
associating entity is polymorphic and has the declared type named in the type
guard statement.
This patch makes sure the associating entity matches the selector if it is
an array, a pointer or an allocatable.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D140017