Subzero: Manage each Cfg as a std::unique_ptr<Cfg>.

src/IceCfg.cpp

Index: src/IceCfg.cpp
diff --git a/src/IceCfg.cpp b/src/IceCfg.cpp
index 53f9a9f76a46f7f0752c349540c246757a378fe5..f2244479a8402aec9080b716aa827ffc55ced4c3 100644
--- a/src/IceCfg.cpp
+++ b/src/IceCfg.cpp
@@ -49,7 +49,7 @@ Cfg::Cfg(GlobalContext *Ctx)
 Cfg::~Cfg() {
   assert(ICE_TLS_GET_FIELD(CurrentCfg) == this);
   // Reset the thread-local CurrentCfg pointer.
-  ICE_TLS_SET_FIELD(CurrentCfg, nullptr);
+  updateTLS(nullptr);
 }
 
 void Cfg::setError(const IceString &Message) {