cgraph: flatten and same_body aliases [PR96078]

cgraph: flatten and same_body aliases [PR96078]

The patch for PR92372 made us start warning about a flatten attribute on an
alias.  But in the case of C++ 'tor base/complete variants, the user didn't
create the alias.  If the alias target also has the attribute, the alias
points to a flattened function, so we shouldn't warn.

gcc/ChangeLog:

	PR c++/96078
	* cgraphunit.c (process_function_and_variable_attributes): Don't
	warn about flatten on an alias if the target also has it.
	* cgraph.h (symtab_node::get_alias_target_tree): New.

gcc/testsuite/ChangeLog:

	PR c++/96078
	* g++.dg/ext/attr-flatten1.C: New test.