[gn build] Port 401b5cc

Author: llvmgnsyncbot

llvm/utils/gn/secondary/lldb/source/Plugins/Language/CPlusPlus/BUILD.gn

@@ -64,6 +64,7 @@ static_library("CPlusPlus") {
     "MsvcStl.cpp",
     "MsvcStlSmartPointer.cpp",
     "MsvcStlTuple.cpp",
+    "MsvcStlVariant.cpp",
     "MsvcStlVector.cpp",
   ]
 }