[CMake] Fix setting result of libatomic check for MSVC

We were skipping the libatomic requirement check for MSVC, but not setting
the corresponding variable, HAVE_CXX_ATOMICS_WITHOUT_LIB. D69869 seems to
have to failed to build on ARM MSVC because of that, and was reverted. This
should probably fix the issue. The plan is to check the result of the build
bots and then submit a more thoroughly refactored version for review.

llvm/cmake/modules/CheckAtomic.cmake

@@ -53,6 +53,8 @@ if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
       message(FATAL_ERROR "Host compiler appears to require libatomic, but cannot find it.")
     endif()
   endif()
+elseif(MSVC)
+  set(HAVE_CXX_ATOMICS_WITHOUT_LIB True)
 endif()
 
 # Check for 64 bit atomic operations.