Disable ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE for clang-cl.

The Lexan and MSVC toolchains both set _MSC_VER. The MSVC toolchain must set `ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE` and `ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE`, in order to use Abseil workarounds in the absence of these types. This is not necessary for clang-cl.

This change excludes clang-cl from the predicate setting these variables.

PiperOrigin-RevId: 465589196
Change-Id: I0426ec4f844aabe7cdde5c60725f6a9a6b16479f

absl/base/config.h

@@ -268,7 +268,7 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
      ((ABSL_INTERNAL_HAVE_MIN_GNUC_VERSION(7, 4) && defined(__GLIBCXX__)) || \
       (ABSL_INTERNAL_HAVE_MIN_GNUC_VERSION(8, 2) &&                          \
        defined(_LIBCPP_VERSION)))) ||                                        \
-    (defined(_MSC_VER) && !defined(__NVCC__))
+    (defined(_MSC_VER) && !defined(__NVCC__) && !defined(__clang__))
 #define ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE 1
 #define ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE 1
 #endif