Split out DwarfFile from DwarfDebug into its own .h/.cpp files.

Some of these types (DwarfDebug in particular) are quite large to begin
with (and I keep forgetting whether DwarfFile is in DwarfDebug or
DwarfUnit... ) so having a few smaller files seems like goodness.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207010 91177308-0d34-0410-b5e6-96231b3b80d8