NFC: DebugInfo: Reduce scope of DebugOffset to simplify code
NFC: DebugInfo: Reduce scope of DebugOffset to simplify code

This was being used as a sort of indirect out parameter from shouldDump
- seems simpler to use it as the actual result of the call. (this does
mean using a pointer to an Optional & actually using all 3 states (null,
None, and present) which is, admittedly, a tad subtle - but given the
limited scope, seems OK to me - open to discussion though, if others
feel strongly about it)

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@346691 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm