More P0202 constexpr work. This commit adds fill/fill_n/generate/generate_n/unique/unique_copy. I removed a specialization of fill_n that recognized when we were dealing with raw pointers and 1 byte trivially-assignable types and did a memset, because the compiler will do that optimization for us.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@323050 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/algorithms/alg.modifying.operations/alg.unique/unique.pass.cpp

@@ -12,7 +12,7 @@
 // template<ForwardIterator Iter>
 //   requires OutputIterator<Iter, Iter::reference>
 //         && EqualityComparable<Iter::value_type>
-//   Iter
+//   constexpr Iter        // constexpr after C++17
 //   unique(Iter first, Iter last);
 
 #include <algorithm>
@@ -22,6 +22,19 @@
 #include "test_macros.h"
 #include "test_iterators.h"
 
+#if TEST_STD_VER > 17
+TEST_CONSTEXPR bool test_constexpr() {
+          int ia[]       = {0, 1, 1, 3, 4};
+    const int expected[] = {0, 1, 3, 4};
+	const size_t N = 4;
+	
+    auto it = std::unique(std::begin(ia), std::end(ia));
+    return it == (std::begin(ia) + N)
+        && std::equal(std::begin(ia), it, std::begin(expected), std::end(expected))
+        ;
+    }
+#endif
+
 template <class Iter>
 void
 test()
@@ -182,4 +195,8 @@ int main()
     test1<random_access_iterator<Ptr*> >();
     test1<Ptr*>();
 #endif
+
+#if TEST_STD_VER > 17
+    static_assert(test_constexpr());
+#endif
 }