Export Mutex::Dtor from shared libraries in NDEBUG mode

When Abseil is built as a shared library (`ABSL_BUILD_DLL`) with `NDEBUG` defined, `Mutex::Dtor()` is currently fully inlined and no symbol is emitted.

Downstream clients building against this shared library without `NDEBUG` defined (e.g., a debug build of an application linking against a system-provided release Abseil DLL) will expect a `Mutex::Dtor()` symbol to exist, ensuring ABI compatibility. Currently, this leads to undefined reference linker errors.

This patch aligns `Mutex::Dtor()` with the existing logic for `Mutex::~Mutex()`: it disables inlining in the header when `ABSL_BUILD_DLL` is active and ensures an empty definition is provided in the source file for these builds.

PiperOrigin-RevId: 828160210
Change-Id: I5ed1fd6757d631e80d6f084d20bfe8e25f6ef253

Author: Abseil Team

absl/synchronization/mutex.cc

@@ -745,7 +745,8 @@ static unsigned TsanFlags(Mutex::MuHow how) {
 Mutex::~Mutex() { Dtor(); }
 #endif
 
-#if !defined(NDEBUG) || defined(ABSL_HAVE_THREAD_SANITIZER)
+#if !defined(NDEBUG) || defined(ABSL_HAVE_THREAD_SANITIZER) || \
+    defined(ABSL_BUILD_DLL)
 void Mutex::Dtor() {
   if (kDebugMode) {
     this->ForgetDeadlockInfo();

absl/synchronization/mutex.h

@@ -1139,12 +1139,13 @@ ABSL_ATTRIBUTE_ALWAYS_INLINE
 inline Mutex::~Mutex() { Dtor(); }
 #endif
 
-#if defined(NDEBUG) && !defined(ABSL_HAVE_THREAD_SANITIZER)
-// Use default (empty) destructor in release build for performance reasons.
-// We need to mark both Dtor and ~Mutex as always inline for inconsistent
-// builds that use both NDEBUG and !NDEBUG with dynamic libraries. In these
-// cases we want the empty functions to dissolve entirely rather than being
-// exported from dynamic libraries and potentially override the non-empty ones.
+#if defined(NDEBUG) && !defined(ABSL_HAVE_THREAD_SANITIZER) && \
+    !defined(ABSL_BUILD_DLL)
+// Under NDEBUG and without TSAN, Dtor is normally fully inlined for
+// performance. However, when building Abseil as a shared library
+// (ABSL_BUILD_DLL), we must provide an out-of-line definition. This ensures the
+// Mutex::Dtor symbol is exported from the DLL, maintaining ABI compatibility
+// with clients that might be built in debug mode and thus expect the symbol.
 ABSL_ATTRIBUTE_ALWAYS_INLINE
 inline void Mutex::Dtor() {}
 #endif