libstdc++: Mark some more algorithms constexpr for C++20

As per P0202.

libstdc++-v3/ChangeLog:

	* include/bits/stl_algo.h (for_each_n): Mark constexpr for C++20.
	(search): Likewise for the overload that takes a searcher.
	* testsuite/25_algorithms/for_each/constexpr.cc: Test constexpr
	std::for_each_n.
	* testsuite/25_algorithms/search/constexpr.cc: Test constexpr
	std::search overload that takes a searcher.

libstdc++-v3/include/bits/stl_algo.h

@@ -3832,6 +3832,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  If `__f` has a return value it is ignored.
   */
   template<typename _InputIterator, typename _Size, typename _Function>
+    _GLIBCXX20_CONSTEXPR
     _InputIterator
     for_each_n(_InputIterator __first, _Size __n, _Function __f)
     {
@@ -4251,6 +4252,7 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
    *  @return @p __searcher(__first,__last).first
   */
   template<typename _ForwardIterator, typename _Searcher>
+    _GLIBCXX20_CONSTEXPR
     inline _ForwardIterator
     search(_ForwardIterator __first, _ForwardIterator __last,
 	   const _Searcher& __searcher)

libstdc++-v3/testsuite/25_algorithms/for_each/constexpr.cc

@@ -34,3 +34,15 @@ test()
 }
 
 static_assert(test());
+
+constexpr bool
+test_n()
+{
+  int tot = 0;
+  auto sum = [&total = tot](int i){ total += i; };
+  auto sum2 = std::for_each_n(ca0.begin(), std::size(ca0)-1, sum);
+
+  return tot == 55;
+}
+
+static_assert(test_n());

libstdc++-v3/testsuite/25_algorithms/search/constexpr.cc

@@ -31,6 +31,10 @@ test()
 				 cam.begin(), cam.end(),
 				 std::equal_to<int>());
 
+  const auto outtt2
+    = std::search(ca0.begin(), ca0.end(),
+		  std::default_searcher(cam.begin(), cam.end()));
+
   return true;
 }