[formatters] Add a libstdcpp formatter for for unordered_map, unordered_set,...

[formatters] Add a libstdcpp formatter for  for unordered_map, unordered_set, unordered_multimap, unordered_multiset

This diff adds a data formatter and tests for libstdcpp's unordered_map, unordered_set, unordered_multimap, unordered_multiset

Reviewed By: wallace

Differential Revision: https://reviews.llvm.org/D113760