Some small C++ cleanup PATCHes

[Jason Merrill]

A few minor things I ran across while working on something else:

1) stabilize_expr didn't know about xvalues.  I think this has no 
practical impact, given the current users of stabilize_expr--when it is 
called by stabilize_call, any reference arguments to the call have 
REFERENCE_TYPE rather than the dereferenced type--but I thought that it 
might be good to fix it for potential future users.

2) build_over_call was unnecessarily duplicating the logic for getting 
the 'this' argument that was previously used to set up argarray.

3) finish_function doesn't need special handling for function_depth > 1 
anymore; the comment talks about cxx_pop_function_context wanting to use 
current_function_decl, but that function no longer exists.

Tested x86_64-pc-linux-gnu, applied to trunk.
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