[OPENMP]Fix PR49698: OpenMP declare mapper causes segmentation fault.

The implicitly generated mappings for allocation/deallocation in mappers
runtime should be mapped as implicit, also no need to clear member_of
flag to avoid ref counter increment. Also, the ref counter should not be
incremented for the very first element that comes from the mapper
function.

Differential Revision: https://reviews.llvm.org/D100673