[gn build] Port 1f72fa2

Author: llvmgnsyncbot

llvm/utils/gn/secondary/llvm/unittests/CodeGen/BUILD.gn

@@ -46,6 +46,7 @@ unittest("CodeGenTests") {
     "TargetOptionsTest.cpp",
     "TestAsmPrinter.cpp",
     "TypeTraitsTest.cpp",
+    "X86MCInstLowerTest.cpp",
   ]
   has_custom_main = true
 }