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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@231530 91177308-0d34-0410-b5e6-96231b3b80d8

dslab-epfl/asap,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,dslab-epfl/asap,dslab-epfl/asap,llvm-mirror/llvm,dslab-epfl/asap,apple/swift-llvm,dslab-epfl/asap,llvm-mirror/llvm,dslab-epfl/asap,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,dslab-epfl/asap,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm