[c++14] Implement missed piece of N3323: use "converted constant" rules

[c++14] Implement missed piece of N3323: use "converted constant" rules
for array bounds, not "integer constant" rules.

For an array bound of class type, this causes us to perform an implicit
conversion to size_t, instead of looking for a unique conversion to
integral or unscoped enumeration type. This affects which cases are
valid when a class has multiple implicit conversion functions to
different types.