Use string_view for fnv64

Summary: Avoid the need for a temporary std::string if fnv64 happens to be called with a C-string literal.

Reviewed By: ot, ilvokhin

Differential Revision: D67716872

fbshipit-source-id: 3e8877955c9d9dbe43db3a97376397df5016d992

folly/hash/Hash.h

@@ -351,7 +351,7 @@ constexpr uint64_t fnv64(
  * @methodset fnv
  */
 inline uint64_t fnv64(
-    const std::string& str, uint64_t hash = fnv64_hash_start) noexcept {
+    std::string_view str, uint64_t hash = fnv64_hash_start) noexcept {
   return fnv64_buf(str.data(), str.size(), hash);
 }