Add default initialization to compressed_pair.

This change introduces the __default_init_tag to memory, and a corresponding
element constructor to allow for default initialization of either of the pair
values. This is useful for classes such as std::string where most (all)
constructors explicitly initialize the values in the constructor.

Patch by Martijn Vels (mvels@google.com)
Reviewed as https://reviews.llvm.org/D70617