Improve StringMap iterator support.

StringMap's iterators did not support LLVM's
iterator_facade_base, which made it unusable in various
STL algorithms or with some of our range adapters.
This patch makes both StringMapConstIterator as well as
StringMapIterator support iterator_facade_base.

With this in place, it is easy to make an iterator adapter
that iterates over only keys, and whose value_type is
StringRef.  So I add StringMapKeyIterator as well, and
provide the method StringMap::keys() that returns a
range that can be iterated.

Differential Revision: https://reviews.llvm.org/D31171

llvm-svn: 298436