[libc++] Provide an assignment operator from pair<U, V> in C++03

This adds an extension to std::pair in C++03 mode where we provide an
assignment operator from a pair<U, V>. Previously, any code trying to
trigger such an assignment operator would have tried using the
`operator=(pair const&)` copy assignment operator, which would then
have tried creating a `pair const&` by using the unconstrained
pair<U, V> constructor.

After this patch, pair will instead go through operator= directly if
its member types are assignable. If they are not assignable, the extension
operator= is disabled with SFINAE and the pair(pair<U, V>) constructor
will be used. Since that constructor is unconstrained, that will either
work or give a hard error.

This should be pretty transparent to users, but this is technically a
behavior change in C++03. Indeed, if a type has both a valid cross-type
assignment operator *and* a valid cross-type constructor, the library
will now prefer the cross-type assignment instead of going through the
cross-type constructor and then using the copy-constructor. Since this
is the mandated behavior in C++11, however, one could argue that any user
broken by that needs to change their code.

The motivation for this change is to allow simplifying the definition
of std::map's value_type, which requires handling assignment to a pair
of references properly. This patch will allow removing complexity from
https://llvm.org/D121485 instead of adding complexity in that patch.

Differential Revision: https://reviews.llvm.org/D150119

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