[Support] Add StringMap::swap() and a default ctor for iterators

This makes StringMap<> more compatible with std::map<std::string, ...>.

Differential Revision: http://llvm-reviews.chandlerc.com/D842

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182487 91177308-0d34-0410-b5e6-96231b3b80d8