[dsymutil] Set linked unit start offset early. NFC.

The start offset of a linked unit is known before starting to clone
its DIEs. Handling DW_FORM_ref_addr attributes requires that this
offset is set while cloning the unit. Split CompileUnit::computeOffsets()
into setStartOffset() and computeNextUnitOffset() and call them
repsectively before cloning the DIEs and right after.

llvm-svn: 231530