DebugInfo: Split DebugLocEntry into its own file.

It seems big enough that it deserves its own file - but it is header
only, so there's no need for another cpp file, etc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205360 91177308-0d34-0410-b5e6-96231b3b80d8