llvm-symbolizer: access the base address from the skeleton CU, not the split unit

llvm-symbolizer: access the base address from the skeleton CU, not the split unit

In Split DWARF, if the unit had a non-trivial base address (a real
low_pc, rather than one with fixed value 0) then computing addresses
needs to access that base address to add to any base address-relative
values. But the code was trying to access the base address in the split
unit, when it's actually in the skeleton unit. So delegate to the
skeleton if it's available.

Fixes #62941