[flang][OpenMP] Handle symbols on composite simd with multiple privatizers (llvm#155640)

In some cases, a clause on a composite simd construct applied to simd
can be using a symbol that is also used by another privatizer, not
applied to simd. Correctly handle this scenario by checking which
directive the privatizer is being generated for while determining
whether to emit the copy region.

Fixes llvm#155195.

Signed-off-by: Kajetan Puchalski <[email protected]>

Author: mrkajetanp