[gn build] Port 14afac0

Author: llvmgnsyncbot

llvm/utils/gn/secondary/llvm/lib/SandboxIR/BUILD.gn

@@ -6,6 +6,7 @@ static_library("SandboxIR") {
     "//llvm/lib/Support",
   ]
   sources = [
+    "Argument.cpp",
     "Context.cpp",
     "Module.cpp",
     "Pass.cpp",