cgraph: Fix up semantic_interposition handling [PR105306]

cgraph_node has a semantic_interposition flag which should mirror
opt_for_fn (decl, flag_semantic_interposition).  But it actually is
initialized not from that, but from flag_semantic_interposition in the
  explicit symtab_node (symtab_type t)
    : type (t), resolution (LDPR_UNKNOWN), definition (false), alias (false),
...
      semantic_interposition (flag_semantic_interposition),
...
      x_comdat_group (NULL_TREE), x_section (NULL)
  {}
ctor.  I think that might be fine for varpool nodes, but since
flag_semantic_interposition is now implied from -Ofast it isn't correct
for cgraph nodes, unless we guarantee that cgraph node for a particular
function decl is always created while that function is
current_function_decl.  That is often the case, but not always as the
following function shows.
Because symtab_node's ctor doesn't know for which decl the cgraph node
is being created, the following patch keeps that as is, but updates it from
opt_for_fn (decl, flag_semantic_interposition) when we know that, or for
clones copies that flag (often it is then overridden in
set_new_clone_decl_and_node_flags, but not always).

2022-04-20  Jakub Jelinek  <jakub@redhat.com>

	PR ipa/105306
	* cgraph.cc (cgraph_node::create): Set node->semantic_interposition
	to opt_for_fn (decl, flag_semantic_interposition).
	* cgraphclones.cc (cgraph_node::create_clone): Copy over
	semantic_interposition flag.

	* g++.dg/opt/pr105306.C: New test.