12.2 states that a temporary bound to a reference lives as long as the 
reference itself.  We have done that for reference variables, but not in 
other cases, such as aggregate initialization of a struct with reference 
members.  In C++11, elements of a std::initializer_list have the same 
semantics; they live as long as the std::initializer_list.  Again, we 
were implementing that for initializer_list variables but not for 
initializer_list subobjects.  This patch fixes that.

Furthermore, if a temporary's lifetime is extended, we need to also 
extend the lifetimes of any temporaries bound to references in its 
initializer, and so on.

The patch introduces a function extend_ref_init_temps called from 
store_init_value after the call to digest_init.  To expose elements of 
an initializer_list to this function, I needed to stop using 
build_aggr_init_full_exprs for aggregate initialization of arrays, and 
consequently needed to call build_vec_init from store_init_value to use 
one EH region for cleaning up the whole array rather than one per 
element.  To deal with multiple extended temporaries, we need to change 
the cleanup pointer from a single tree to a VEC, and add a discriminator 
to the mangled name of reference init temporaries; this has no ABI 
impact, since the temporaries have no linkage, but I also updated the 
demangler accordingly.

Since we now do lifetime extension in extend_ref_init_temps, we can 
drastically simplify initialize_reference and do away with 
build_init_list_var_init.

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