give StringMap a new ctor which allows you to initialize it

give StringMap a new ctor which allows you to initialize it
with an existing allocator.  The interesting use case of this
is that it allows "StringMap<whatever, BumpPtrAllocator&>" for
when you want to allocate out of a preexisting bump pointer
allocator owned by someone else.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@109213 91177308-0d34-0410-b5e6-96231b3b80d8