Subzero: Fix a build error. Also reformat.

src/IceMemory.h

Index: src/IceMemory.h
diff --git a/src/IceMemory.h b/src/IceMemory.h
index 366c49d0686ab712aa1ebf628abd8443dad8c4fd..b651d0a2b66cf7f0f4e732c78c6f692b608de984 100644
--- a/src/IceMemory.h
+++ b/src/IceMemory.h
@@ -144,7 +144,7 @@ public:
   static allocator_type current();
   static void set_current(const manager_type *Manager);
   static void set_current(ArenaAllocator *Allocator);
-  static void set_current(nullptr_t);
+  static void set_current(std::nullptr_t);
 
 private:
   ICE_TLS_DECLARE_FIELD(ArenaAllocator *, CfgAllocator);