Mark non-modifying container algorithms as constexpr for C++20.

This change marks Abseil's non-modifying sequence operations including
absl::linear_search and absl::c_linear_search as constexpr when building with
C++20.
PiperOrigin-RevId: 659812405
Change-Id: I8dc2cee873f30531b2eb8fb3da12085505a43a1a

absl/algorithm/algorithm.h

@@ -53,8 +53,8 @@ using std::rotate;
 // n = (`last` - `first`) comparisons. A linear search over short containers
 // may be faster than a binary search, even when the container is sorted.
 template <typename InputIterator, typename EqualityComparable>
-bool linear_search(InputIterator first, InputIterator last,
-                   const EqualityComparable& value) {
+ABSL_INTERNAL_CONSTEXPR_SINCE_CXX20 bool linear_search(
+    InputIterator first, InputIterator last, const EqualityComparable& value) {
   return std::find(first, last, value) != last;
 }