Fix libcxx formatters for changes in r300140.

Summary:
LLVM r300140 changed the layout and field names of __compressed_pair, which
broke LLDB's std::vector, std::map and std::unsorted_map formatters.

This patch attempts to fix these formatters by having them interogate the
__compressed_pair values to determine whether they're pre- or post-r300140
variants, then access them accordingly.

Reviewers: jingham, EricWF

Reviewed By: jingham

Differential Revision: https://reviews.llvm.org/D32554

llvm-svn: 301493