Rename DWARFContext::getLineTableForCompileUnit to getLineTableForUnit.

All DWARFUnits can have line information. Rename and change arguments' type
accordingly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217130 91177308-0d34-0410-b5e6-96231b3b80d8