[AMDGPU] Fixed MSVC build break
[AMDGPU] Fixed MSVC build break

Error was:

field of type 'llvm::ArgDescriptor' has private default constructor
const AMDGPUFunctionArgInfo AMDGPUArgumentUsageInfo::ExternFunctionInfo{};
                                                                        ^

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@310048 91177308-0d34-0410-b5e6-96231b3b80d8

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