PATCH to strip unnecessary TARGET_EXPRs for c++/27371

[Jason Merrill <jason at redhat dot com>]

The C++ front end automatically wraps all calls to functions that return 
a class with a TARGET_EXPR in order to track the temporary object 
returned from the function.  This becomes a MODIFY_EXPR during 
gimplification, and makes it difficult to see that the return value is 
in fact never used.

This patch strips such TARGET_EXPRs in convert_to_void and fixes the 
warning.  The temporaries will now be allocated in expand_call or 
declare_return_variable (which needed some adjustment to handle 
allocating a return slot in the case of DECL_BY_REFERENCE).

I hoped that shifting these unused temporaries to being allocated at RTL 
conversion time through assign_temp would also help with 25505, but in 
fact it doesn't have any effect; the assign_temp temporaries aren't 
currently shared any better than gimple temporaries.  But it should be 
possible to improve that; there should be no reason for an assign_temp 
temporary to live longer than a single gimple statement, anything that 
needs to live longer should already be represented in the gimple.

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