[CodeGen] Fix build bots which uses old Clang broken in r314046. (NFC)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314049 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/InterferenceCache.h

@@ -38,7 +38,7 @@ class LLVM_LIBRARY_VISIBILITY InterferenceCache {
     SlotIndex First;
     SlotIndex Last;
 
-    BlockInterference() = default;
+    BlockInterference() {}
   };
 
   /// Entry - A cache entry containing interference information for all aliases