Move DwarfCompileUnit from DwarfUnit.h to its own header (DwarfCompileUnit.h)

Move DwarfCompileUnit from DwarfUnit.h to its own header (DwarfCompileUnit.h)

In preparation for sinking all the subprogram emission code down from
DwarfDebug into DwarfCompileUnit, this will avoid bloating
DwarfUnit.h/cpp greatly and make concerns a bit more clear/isolated.

(sinking this handling down is part of the work to handle emitting
minimal subprograms for -gmlt-like data into the skeleton CU under
fission)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219057 91177308-0d34-0410-b5e6-96231b3b80d8