IR: Use DenseSet instead of DenseMap for ConstantUniqueMap; NFC

Use a DenseSet instead of a DenseMap for constants in LLVMContextImpl.
Last time I looked at this was some time before r223588, when
DenseSet<V> had no advantage over DenseMap<V,char>.  After r223588,
there's a 50% memory savings.

This is all mechanical.  There were little bits of missing API from
DenseSet so I added the trivial implementations:

  - iterator::operator++(int)
  - template <class LookupKeyT> insert_as(ValueTy, LookupKeyT)

There should be no functionality change, just reduced memory consumption
(this wasn't on a profile or anything; just a cleanup I stumbled on).

llvm-svn: 265577