RFA: cgraph PATCH for c++/51812 (link failure with undefined/unneeded virtual function)

[Jason Merrill]

In this testcase, main doesn't call anything, so the inline functions in 
the classes can be optimized away, and we don't need the vtables or the 
virtual functions they refer to.  But cgraph_decide_is_function_needed 
wants to write out all static functions at -O0, which in this case was 
including the local thunk alias created by use_thunk.  Clearly we don't 
want to force such an alias out when it isn't needed, so I've added 
same-body aliases to the list of things that we don't need to emit.

OK for trunk?
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