[IR] Add DenseMapInfo<CallSite>.

Summary:
A CallSite is basically an Instruction*, and you can put Instruction*s
into DenseMaps, so you should be able to do the same with CallSites.

This is used in a later patch.

Reviewers: timshen

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25643

llvm-svn: 284870