Rename StringMap::emplace_second to try_emplace.

Coincidentally this function maps to the C++17 try_emplace. Rename it
for consistentcy with C++17 std::map. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@276276 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed