Make StringSet::insert return pair<iterator, bool> like other self-associative containers

StringSet is still a bit dodgy in that it exposes the raw iterator of
the StringMap parent, which exposes the weird detail that StringSet
actually has a 'value'... but anyway, this is useful for a handful of
clients that want to reference the newly inserted/persistent string data
in the StringSet/Map/Entry/thing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222302 91177308-0d34-0410-b5e6-96231b3b80d8