[flang][OpenMP] Move handling of OpenMP symbol flags to OpenMP.cpp (#75523)

The function `instantiateVariable` in Bridge.cpp has the following code:
```
  if (var.getSymbol().test(
          Fortran::semantics::Symbol::Flag::OmpThreadprivate))
    Fortran::lower::genThreadprivateOp(*this, var);

  if (var.getSymbol().test(
          Fortran::semantics::Symbol::Flag::OmpDeclareTarget))
    Fortran::lower::genDeclareTargetIntGlobal(*this, var);
```

Implement `handleOpenMPSymbolProperties` in OpenMP.cpp, move the above
code there, and have `instantiateVariable` call this function instead.

This would further separate OpenMP-related details into OpenMP.cpp.