Remove implicit conversion from __value_type to value_type in [unordered_][multi]map.  This fixes http://llvm.org/bugs/show_bug.cgi?id=16549

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@185711 91177308-0d34-0410-b5e6-96231b3b80d8