openmp: Copy DECL_LANG_SPECIFIC and DECL_LANG_FLAG_? to tree-nested decl copy [PR114825]

tree-nested.cc creates in 2 spots artificial VAR_DECLs, one of them is used
both for debug info and OpenMP/OpenACC lowering purposes, the other solely for
OpenMP/OpenACC lowering purposes.
When the decls are used in OpenMP/OpenACC lowering, the OMP langhooks (mostly
Fortran, C just a little and C++ doesn't have nested functions) then inspect
the flags on the vars and based on that decide how to lower the corresponding
clauses.

Unfortunately we weren't copying DECL_LANG_SPECIFIC and DECL_LANG_FLAG_?, so
the langhooks made decisions on the default flags on those instead.
As the original decl isn't necessarily a VAR_DECL, could be e.g. PARM_DECL,
using copy_node wouldn't work properly, so this patch just copies those
flags in addition to other flags it was copying already.  And I've removed
code duplication by introducing a helper function which does copying common
to both uses.

2024-04-25  Jakub Jelinek  <jakub@redhat.com>

	PR fortran/114825
	* tree-nested.cc (get_debug_decl): New function.
	(get_nonlocal_debug_decl): Use it.
	(get_local_debug_decl): Likewise.

	* gfortran.dg/gomp/pr114825.f90: New test.