Add DwarfCompileUnit::BaseAddress to track the base address used by relative addressi...

Add DwarfCompileUnit::BaseAddress to track the base address used by relative addressing in debug_ranges and debug_loc

This is one of a few steps to generalize range handling to include the
CU range (thus the CU's range list will be moved into the range list
list, losing track of the base address in the process), which means
generalizing ranges from both the skeleton and full unit under fission.

And... then I can used that generalized support for ranges in
fission+gmlt where there'll be a bunch more ranges in the skeleton.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221182 91177308-0d34-0410-b5e6-96231b3b80d8