[gn build] Port 7a2c5c6

Author: llvmgnsyncbot

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

@@ -13,6 +13,7 @@ static_library("SandboxIR") {
     "SandboxIR.cpp",
     "Tracker.cpp",
     "Type.cpp",
+    "User.cpp",
     "Value.cpp",
   ]
 }