[flang][openacc] Avoid privatizing symbols during semantics (#69506)

During flang handling of semantics of OpenACC private/firstprivate/
reduction clauses (including the implicitly private loop IV), a new
scoped symbol was being created. This could lead to ambiguity in the
lowered FIR - aka having multiple fir.declare for the same symbol.
Because lowering of OpenACC does not materialize the meaning of the
private clauses (by actually creating a scoped local symbol), it does
not make sense to create a new symbol in semantics either.

I updated the acc-symbols01.f90 test to reflect this updated approach.
Technically, the test could be removed, but it made sense to keep in
place to highlight this intentional decision.