[Flang][OpenMP] Fix COPYIN of derived types with allocatable components at -O3 (#196063)

COPYIN of threadprivate derived types with allocatable components
segfaults at -O3 because the OpenMP runtime zero-fills per-thread
storage, leaving allocatable component descriptors with invalid
metadata. This patch skips the copy on the master thread (where source
and destination alias) and uses temporary_lhs assignment on worker
threads so the runtime initializes descriptors before the deep copy.

Assisted-by: Claude Opus 4.6

Fixes :
[https://github.com/llvm/llvm-project/issues/196134](https://github.com/llvm/llvm-project/issues/196134)
Minimal reprducing test-case : 
```
program repro_o3_segv
  use omp_lib
  implicit none

  integer, parameter :: NT = 4
  integer :: i, rc
  integer :: nThreads(NT), nThreads1(NT)

  type structure_1
    integer, allocatable :: a(:)
  end type
  type structure_2
    type(structure_1) :: struc1
  end type

  type(structure_2), save :: struc2(2)
!$omp threadprivate(struc2)

  rc = 0
  nThreads  = -999
  nThreads1 = -999

  ! Keep this: dynamic teams can change codegen/runtime behavior.
  call omp_set_dynamic(.true.)
  call omp_set_num_threads(NT)

  allocate(struc2(1)%struc1%a(2))
  struc2(1)%struc1%a(1) = 1
  struc2(1)%struc1%a(2) = 2

!$omp parallel copyin(struc2)
  if (omp_get_thread_num() == NT-1) then
    ! Keep branch shape from original.
    ! struc2(1)%struc1%a(1) = 3
  end if
!$omp barrier

  allocate(struc2(2)%struc1%a(3))
  nThreads(omp_get_thread_num()+1) = struc2(1)%struc1%a(1)
  struc2(2)%struc1%a(1) = omp_get_thread_num()+1
  struc2(2)%struc1%a(2) = omp_get_thread_num()+2
!$omp end parallel

  do i = 1, NT
    if (nThreads(i) /= 1) rc = 1
  end do

  struc2(2)%struc1%a(2) = -1

!$omp parallel copyin(struc2)
  nThreads(omp_get_thread_num()+1)  = struc2(2)%struc1%a(1)
  nThreads1(omp_get_thread_num()+1) = struc2(2)%struc1%a(2)
!$omp end parallel

  do i = 1, NT
    if (nThreads1(i) /= -1) rc = 1
  end do

  if (rc /= 0) stop 1

  deallocate(struc2(1)%struc1%a)
  deallocate(struc2(2)%struc1%a)
end program
```
[>./flang -fopenmp -fopenmp-version=50 copyin_derrived_alloct.f90 -O3
> ./a.out 
Segmentation fault (core dumped)

Co-authored-by: Chandra Ghale <ghale@pe34genoa.hpc.amslabs.hpecorp.net>