[codeview] Fully initialize DataSym when mapping from YAML
[codeview] Fully initialize DataSym when mapping from YAML

In the object file, the section index and relative offset are typically
zero, so make these YAML fields optional with a default.

It looks like there may be more partially initialized symbol records,
but this should fix the msan bot.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@305842 91177308-0d34-0410-b5e6-96231b3b80d8

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