[ADT] Add StringMap::insert_or_assign

Summary: Similar to std::unordered_map::insert_or_assign

Reviewers: alexshap, bkramer, dblaikie, lhames

Subscribers: jkorous, dexonsmith, kristina, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67668

llvm-svn: 372813