Replace llvm::integer_sequence and friends with the C++14 standard version

The implementation in libc++ takes O(1) compile time, ours was O(n).

llvm-svn: 368990
diff --git a/clang/lib/CodeGen/EHScopeStack.h b/clang/lib/CodeGen/EHScopeStack.h
index 3b0db35..0ed67aa 100644
--- a/clang/lib/CodeGen/EHScopeStack.h
+++ b/clang/lib/CodeGen/EHScopeStack.h
@@ -199,14 +199,14 @@
     SavedTuple Saved;
 
     template <std::size_t... Is>
-    T restore(CodeGenFunction &CGF, llvm::index_sequence<Is...>) {
+    T restore(CodeGenFunction &CGF, std::index_sequence<Is...>) {
       // It's important that the restores are emitted in order. The braced init
       // list guarantees that.
       return T{DominatingValue<As>::restore(CGF, std::get<Is>(Saved))...};
     }
 
     void Emit(CodeGenFunction &CGF, Flags flags) override {
-      restore(CGF, llvm::index_sequence_for<As...>()).Emit(CGF, flags);
+      restore(CGF, std::index_sequence_for<As...>()).Emit(CGF, flags);
     }
 
   public: