openmp: Fix up cloning of dynamic C++ initializers for OpenMP target [PR119370]

openmp: Fix up cloning of dynamic C++ initializers for OpenMP target [PR119370]

The following testcase ICEs, because we emit the dynamic initialization twice,
once for host and once for target initialization, and although we use
copy_tree_body_r to unshare it, e.g. for the array initializers it can contain
TARGET_EXPRs with local temporaries (or local temporaries alone).
Now, these temporaries were created when current_function_decl was NULL,
they are outside of any particular function, so they have DECL_CONTEXT NULL.
That is normally fine, gimple_add_tmp_var will set DECL_CONTEXT for them
later on to the host __static_init* function into which they are gimplified.
The problem is that the copy_tree_body_r cloning happens before that (and has
to, gimplification is destructive and so we couldn't gimplify the same tree
again in __omp_static_init* function) and uses auto_var_in_fn_p to see what
needs to be remapped.  But that means it doesn't remap temporaries with
NULL DECL_CONTEXT and having the same temporaries in two different functions
results in ICEs (sure, one can e.g. use parent function's temporaries in a
nested function).

The following patch just arranges to set DECL_CONTEXT on the temporaries
to the host dynamic initialization function, so that they get remapped.
If gimplification cared whether DECL_CONTEXT is NULL or not, we could
remember those that we've changed in a vector and undo it afterwards,
but seems it doesn't really care.

2025-03-20  Jakub Jelinek  <jakub@redhat.com>

	PR c++/119370
	* decl2.cc (set_context_for_auto_vars_r): New function.
	(emit_partial_init_fini_fn): Call walk_tree with that function
	on &init before walk_tree with copy_tree_body_r.

	* g++.dg/gomp/pr119370.C: New test.