Repair fallthrough was broken by d4493165 (#5115) (#5139)

- GCC: fixed broken `fallthrough` (checked with 7.3 and 8.2)
- Clang: added `fallthrough` support
- Clang: added `-Wimplicit-fallthrough` checking

src/idl_gen_rust.cpp

@@ -130,6 +130,7 @@ FullType GetFullType(const Type &type) {
       case ftUnionKey:
       case ftUnionValue: {
         FLATBUFFERS_ASSERT(false && "vectors of unions are unsupported");
+        break;
       }
       default: {
         FLATBUFFERS_ASSERT(false && "vector of vectors are unsupported");