Improve use of CfgLocalAllocator and introduce containers that use it.

src/IceGlobalContext.cpp

Index: src/IceGlobalContext.cpp
diff --git a/src/IceGlobalContext.cpp b/src/IceGlobalContext.cpp
index 7b7183d5540852496e2ff9a1b0b256a9635aa61f..c7497da83df5a8978fdcdbfdd1fba6045acb1c70 100644
--- a/src/IceGlobalContext.cpp
+++ b/src/IceGlobalContext.cpp
@@ -722,10 +722,8 @@ GlobalContext::~GlobalContext() {
   llvm::DeleteContainerPointers(AllThreadContexts);
   LockedPtr<DestructorArray> Dtors = getDestructors();
   // Destructors are invoked in the opposite object construction order.
-  for (auto DtorIter = Dtors->crbegin(); DtorIter != Dtors->crend();
-       ++DtorIter) {
-    (*DtorIter)();
-  }
+  for (const auto &Dtor : reverse_range(*Dtors))
+    Dtor();
 }
 
 // TODO(stichnot): Consider adding thread-local caches of constant pool entries