Debug Info: Reorder accessor to match field order.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182816 91177308-0d34-0410-b5e6-96231b3b80d8
Debug Info: Update documentation to match recent (& not so recent) schema changes

This updates the debug info metadata schema documentation for various
schema changes made recently surrounding filename information for
scopes and the representation of imported entities.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182817 91177308-0d34-0410-b5e6-96231b3b80d8