Make `SanitizerSafeCopy()` constexpr, and check for constant evaluation

PiperOrigin-RevId: 504728034
Change-Id: Ifb338247b7484426e25a58580783a1d70d27e6fd

absl/strings/internal/cord_internal.h

@@ -768,7 +768,8 @@ class InlineData {
     }
 
 #ifdef ABSL_INTERNAL_CORD_HAVE_SANITIZER
-    Rep SanitizerSafeCopy() const {
+    constexpr Rep SanitizerSafeCopy() const {
+      if (!absl::is_constant_evaluated()) {
         Rep res;
         if (is_tree()) {
           res = *this;
@@ -777,9 +778,12 @@ class InlineData {
           memcpy(res.as_chars(), as_chars(), inline_size());
         }
         return res;
+      } else {
+        return *this;
+      }
     }
 #else
-    const Rep& SanitizerSafeCopy() const { return *this; }
+    constexpr const Rep& SanitizerSafeCopy() const { return *this; }
 #endif
 
     // If the data has length <= kMaxInline, we store it in `data`, and