[NFC][OpenMP][Flang] Add test for OpenMP target parallel do

Added test which proves that end-to-end compilation of
omp target parallel do costruct is successful for Flang compiler.

Author: DominikAdamski

openmp/libomptarget/test/offloading/fortran/basic-target-parallel-do.f90

@@ -0,0 +1,31 @@
+! Basic offloading test with a target region
+! REQUIRES: flang
+! UNSUPPORTED: nvptx64-nvidia-cuda-LTO
+! UNSUPPORTED: aarch64-unknown-linux-gnu
+! UNSUPPORTED: aarch64-unknown-linux-gnu-LTO
+! UNSUPPORTED: x86_64-pc-linux-gnu
+! UNSUPPORTED: x86_64-pc-linux-gnu-LTO
+
+! RUN: %libomptarget-compile-fortran-run-and-check-generic
+program main
+   use omp_lib
+   integer :: x(100)
+   integer :: errors = 0
+   integer :: i
+
+   !$omp target parallel do map(from: x)
+   do i = 1, 100
+       x(i) = i
+   end do
+   !$omp end target parallel do
+   do i = 1, 100
+       if ( x(i) .ne. i ) then
+           errors = errors + 1
+       end if
+   end do
+
+   print *,"number of errors: ", errors
+
+end program main
+
+! CHECK: number of errors: 0