[libcxx] Remove the "reduced-arity-initialization" extension from the uses-allocator constructors

Summary:
A default uses-allocator constructor has been added since that overload was previously provided by the extended constructor.

Since Clang does implicit conversion checking after substitution this constructor has to deduce the allocator_arg_t parameter so that it can prevent the evaluation of "is_default_constructible" if the first argument doesn't match. See http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1391 for more information.

This patch fixes PR24779 (https://llvm.org/bugs/show_bug.cgi?id=24779)

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D19006

llvm-svn: 266409