cgraphclones: Don't share DECL_ARGUMENTS between thunk and its artificial thunk ...

cgraphclones: Don't share DECL_ARGUMENTS between thunk and its artificial thunk [PR108854]

The following testcase ICEs on x86_64-linux with -m32.  The problem is
we create an artificial thunk and because of -fPIC, ia32 and thunk
destination which doesn't bind locally can't use a mi thunk.
The ICE is because during expansion to RTL we see SSA_NAME for a PARM_DECL,
but the PARM_DECL doesn't have DECL_CONTEXT of the current function.
This is because duplicate_thunk_for_node creates a new DECL_ARGUMENTS chain
only if some arguments need modification.

The following patch fixes it by copying the DECL_ARGUMENTS list even if
the arguments can stay as is, to update DECL_CONTEXT on them.  While for
mi thunks it doesn't really matter because we don't use those arguments
in any way, for other thunks it is important.

2023-02-23  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/108854
	* cgraphclones.cc (duplicate_thunk_for_node): If no parameter
	changes are needed, copy at least DECL_ARGUMENTS PARM_DECL
	nodes and adjust their DECL_CONTEXT.

	* g++.dg/opt/pr108854.C: New test.