Add further diagnostics under clang for string_view(nullptr)

Detection isn't perfect, but it is better than nothing:
https://godbolt.org/z/YzeMeb58j

PiperOrigin-RevId: 837204651
Change-Id: Id1027c4c27bd95ad923e4c5d242a28079b16db79

absl/strings/string_view.h

@@ -58,6 +58,15 @@ ABSL_NAMESPACE_END
 
 #else  // ABSL_USES_STD_STRING_VIEW
 
+#if ABSL_HAVE_ATTRIBUTE(diagnose_if)
+#define ABSL_INTERNAL_DIAGNOSE_IF_NULLPTR(x) \
+  __attribute__((diagnose_if(                \
+      x == nullptr,                          \
+      "null passed to a callee that requires a non-null argument", "error")))
+#else
+#define ABSL_INTERNAL_DIAGNOSE_IF_NULLPTR(x)
+#endif
+
 #if ABSL_HAVE_BUILTIN(__builtin_memcmp) ||        \
     (defined(__GNUC__) && !defined(__clang__)) || \
     (defined(_MSC_VER) && _MSC_VER >= 1928)
@@ -225,7 +234,7 @@ class ABSL_ATTRIBUTE_VIEW string_view {
   // instead (see below).
   // The length check is skipped since it is unnecessary and causes code bloat.
   constexpr string_view(  // NOLINT(runtime/explicit)
-      const char* absl_nonnull str)
+      const char* absl_nonnull str) ABSL_INTERNAL_DIAGNOSE_IF_NULLPTR(str)
       : ptr_(str), length_(str ? StrlenInternal(str) : 0) {
     ABSL_HARDENING_ASSERT(str != nullptr);
   }
@@ -779,6 +788,7 @@ std::ostream& operator<<(std::ostream& o, string_view piece);
 ABSL_NAMESPACE_END
 }  // namespace absl
 
+#undef ABSL_INTERNAL_DIAGNOSE_IF_NULLPTR
 #undef ABSL_INTERNAL_STRING_VIEW_MEMCMP
 
 #endif  // ABSL_USES_STD_STRING_VIEW