[libcxx] [test] Fix test bugs in string.cons/copy_alloc.pass.cpp.

[libcxx] [test] Fix test bugs in string.cons/copy_alloc.pass.cpp.

Fixed the inability to properly rebind the testing allocator, by making the
inner alloc_impl type a plain struct and making the operations templates. Before
rebind failed to compile complaining that a alloc_impl<T>* was not convertible
to an alloc_impl<U>*.

This enables the test to pass for MSVC++ once we provide the strong guarantee
for the copy assignment operator.

Reviewed as https://reviews.llvm.org/D60023

llvm-svn: 357545