[MC] Fix -Wmissing-field-initializer warning after r329067.
[MC] Fix -Wmissing-field-initializer warning after r329067.

This should fix the problem reported by the lld buildbots:
 - Builder lld-x86_64-darwin13, Build #19782
 - Builder lld-perf-testsuite, Build #1419


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@329068 91177308-0d34-0410-b5e6-96231b3b80d8

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