C++ PATCH for c++/53816 (ICE with local reference in template)

[Jason Merrill]

resolves_to_fixed_type_p has caused a lot of ICEs in templates over the 
years because it wants to look inside the DECL_INITIAL of variables with 
non-constant initializers which are still in pre-tsubst form.  Checking 
processing_template_decl to prevent this isn't strong enough, because 
fold_non_dependent_expr clears processing_template_decl in order to do 
its tsubst folding.  But instead we can test whether the current 
function is not completely instantiated.

Tested x86_64-pc-linux-gnu, applying to trunk and 4.7 (more conservative 
version).
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