Make `ABSL_ATTRIBUTE_NO_UNIQUE_ADDRESS` work on MSVC

https://devblogs.microsoft.com/cppblog/msvc-cpp20-and-the-std-cpp20-switch/#c20-no_unique_address
MSVC disables [[no_unique_address]] by default because of ABI
compatibility. Since Abseil does not claim ABI compatibility in
mixed-mode builds, we can offer it unconditionally.

Fixes #1418

PiperOrigin-RevId: 518894036
Change-Id: If7653e65703b335783f11d296de7e32fc35d4fc6

absl/base/attributes.h

@@ -787,7 +787,15 @@
 // For code that is assured to only build with C++20 or later, prefer using
 // the standard attribute `[[no_unique_address]]` directly instead of this
 // macro.
-#if ABSL_HAVE_CPP_ATTRIBUTE(no_unique_address)
+//
+// https://devblogs.microsoft.com/cppblog/msvc-cpp20-and-the-std-cpp20-switch/#c20-no_unique_address
+// Current versions of MSVC have disabled `[[no_unique_address]]` since it
+// breaks ABI compatibility, but offers `[[msvc::no_unique_address]]` for
+// situations when it can be assured that it is desired. Since Abseil does not
+// claim ABI compatibility in mixed builds, we can offer it unconditionally.
+#if defined(_MSC_VER) && _MSC_VER >= 1929
+#define ABSL_ATTRIBUTE_NO_UNIQUE_ADDRESS [[msvc::no_unique_address]]
+#elif ABSL_HAVE_CPP_ATTRIBUTE(no_unique_address)
 #define ABSL_ATTRIBUTE_NO_UNIQUE_ADDRESS [[no_unique_address]]
 #else
 #define ABSL_ATTRIBUTE_NO_UNIQUE_ADDRESS