[flang][OpenMP] Support lowering of threadprivate directive for non global variables

Non-global variable which can be in threadprivate directive must be one
variable in main program, and it has implicit SAVE attribute. Take it as
with SAVE attribute, so to create GlobalOp for it to simplify the
translation to LLVM IR.

Reviewed By: NimishMishra

Differential Revision: https://reviews.llvm.org/D127047