`internal/log_message`: Use `if constexpr` instead of SFINAE for `operator<<`

SFINAE is harder to read and it makes mangled names longer.

PiperOrigin-RevId: 729591470
Change-Id: I8d254fdd02b993580c7a4df8f477ef4fa11b6f76

absl/log/internal/log_message.h

diff --git a/absl/log/internal/log_message.h b/absl/log/internal/log_message.h
index 7d0e403..4986e7e 100644
--- a/absl/log/internal/log_message.h
+++ b/absl/log/internal/log_message.h
@@ -183,16 +183,9 @@
   LogMessage& operator<<(char (&buf)[SIZE]) ABSL_ATTRIBUTE_NOINLINE;
 
   // Types that support `AbslStringify()` are serialized that way.
-  template <typename T,
-            typename std::enable_if<absl::HasAbslStringify<T>::value,
-                                    int>::type = 0>
-  LogMessage& operator<<(const T& v) ABSL_ATTRIBUTE_NOINLINE;
-
   // Types that don't support `AbslStringify()` but do support streaming into a
   // `std::ostream&` are serialized that way.
-  template <typename T,
-            typename std::enable_if<!absl::HasAbslStringify<T>::value,
-                                    int>::type = 0>
+  template <typename T>
   LogMessage& operator<<(const T& v) ABSL_ATTRIBUTE_NOINLINE;
 
   // Note: We explicitly do not support `operator<<` for non-const references
@@ -308,21 +301,16 @@
 };
 
 // Note: the following is declared `ABSL_ATTRIBUTE_NOINLINE`
-template <typename T,
-          typename std::enable_if<absl::HasAbslStringify<T>::value, int>::type>
+template <typename T>
 LogMessage& LogMessage::operator<<(const T& v) {
-  StringifySink sink(*this);
-  // Replace with public API.
-  AbslStringify(sink, v);
-  return *this;
-}
-
-// Note: the following is declared `ABSL_ATTRIBUTE_NOINLINE`
-template <typename T,
-          typename std::enable_if<!absl::HasAbslStringify<T>::value, int>::type>
-LogMessage& LogMessage::operator<<(const T& v) {
-  OstreamView view(*data_);
-  view.stream() << log_internal::NullGuard<T>().Guard(v);
+  if constexpr (absl::HasAbslStringify<T>::value) {
+    StringifySink sink(*this);
+    // Replace with public API.
+    AbslStringify(sink, v);
+  } else {
+    OstreamView view(*data_);
+    view.stream() << log_internal::NullGuard<T>().Guard(v);
+  }
   return *this;
 }