Add -Wimplicit-fallthrough to fbgemm (#1802)

Summary:
Ensure that we don't accidentally fallthrough switch statements.

Pull Request resolved: #1802

Reviewed By: q10, brad-mengchi

Differential Revision: D46377516

fbshipit-source-id: 509fd7007fb90d8dbe8d4522eaf052acc8a15bd4

CMakeLists.txt

@@ -185,6 +185,7 @@ else(MSVC)
   string(APPEND CMAKE_CXX_FLAGS " -Wextra")
   string(APPEND CMAKE_CXX_FLAGS " -Werror")
   string(APPEND CMAKE_CXX_FLAGS " -Wno-deprecated-declarations")
+  string(APPEND CMAKE_CXX_FLAGS " -Wimplicit-fallthrough")
   target_compile_options(fbgemm_avx2 PRIVATE
     "-m64" "-mavx2" "-mf16c" "-mfma")
   target_compile_options(fbgemm_avx512 PRIVATE