c++: Fix cp_genericize_target_expr for TARGET_EXPRs created for global initialization...

c++: Fix cp_genericize_target_expr for TARGET_EXPRs created for global initialization [PR104031]

The following patch is miscompiled, cp_genericize_target_expr expects
that for the constant part split_nonconstant_init will emit an INIT_EXPR
that will initialize it, but that doesn't happen and instead we get
DECL_INITIAL on the TARGET_EXPR_SLOT that isn't initialized anywhere
in the IL.

The problem is that the TARGET_EXPR has been created while
current_function_decl was NULL, it is inside a global var initializer.
That means the build_local_temp created VAR_DECL has NULL DECL_CONTEXT.
Later on when genericizing the ssdf (current_function_decl is already
non-NULL), the new cp_genericize_target_expr is called and during
split_nonconstant_init it checks is_local_temp, but that due to the NULL
DECL_CONTEXT returns false.  DECL_CONTEXT is set only later on during
gimplification.

The following patch fixes it by setting DECL_CONTEXT also inside of
cp_genericize_target_expr, which fixes the testcase.  But if there are
better spots to do that, please let me know...

2022-01-17  Jakub Jelinek  <jakub@redhat.com>

	PR c++/104031
	* cp-gimplify.c (cp_genericize_target_expr): Set DECL_CONTEXT of
	TARGET_EXPR_SLOT to current_function_decl if it was NULL.

	* g++.dg/cpp1y/pr104031.C: New test.