C++ PATCH for c++/64899 (constexpr default-initialized array)

[Jason Merrill]

build_vec_init works to evaluate constant values for elements of a 
vector initializer, but it wasn't working properly for a 
default-initialized array with a constexpr default constructor.  First I 
needed to add that case to the try_const condition; then I needed to 
change tho form of the initialization because when building the 
constructor call to refer to *iterator, constexpr evaluation saw that 
'iterator' isn't constant and gave up.  To completely solve this for 
cases where we refer to the address of the object or the value of a 
previous element we should hand the whole initialization loop to 
constexpr evaluation, but just using the value-init path fixes this 
testcase.

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