[libc++] Fix stack_allocator

Summary:
To quote STL the problems with stack allocator are"

>"stack_allocator<T, N> is seriously nonconformant to N4582 17.6.3.5 [allocator.requirements].
> First, it lacks a rebinding constructor. (The nested "struct rebind" isn't sufficient.)
> Second, it lacks templated equality/inequality.
> Third, it completely ignores alignment.
> Finally, and most severely, the Standard forbids its existence. Allocators are forbidden from returning memory "inside themselves". This requirement is implied by the Standard's requirements for rebinding and equality. It's permitted to return memory from a separate buffer object on the stack, though."

This patch attempts to address all of those issues.

First, instead of storing the buffer inside the allocator I've change `stack_allocator` to accept the buffer as an argument.

Second, in order to fix rebinding I changed the parameter list from `<class T, size_t NumElements>` to `<class T, size_t NumBytes>`. This allows allocator rebinding
between types that have different sizes. 

Third, I added copy and rebinding constructors and assignment operators.

And finally I fixed the allocation logic to always return properly aligned storage.



Reviewers: mclow.lists, howard.hinnant, STL_MSFT

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D25154

llvm-svn: 283631