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