[TableGen] Make better use of std::map::emplace and emplace construct the object in the map rather than moving it into it. Remove a use std::map::find by remembering the return from emplace.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@334380 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed