When destroying a StringMap, just iterate over the map and destroy the contained elements.  Don't reset them back to 0 as their values aren't needed any more.  This results in ~StringMap() being mostly empty for POD types in BumpPtrAllocators

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204204 91177308-0d34-0410-b5e6-96231b3b80d8