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_general.cpp

@@ -306,7 +306,7 @@ class GeneralGenerator : public BaseGenerator {
       case BASE_TYPE_UNION:
         // Unions in C# use a generic Table-derived type for better type safety
         if (lang_.language == IDLOptions::kCSharp) return "TTable";
-        // fall through
+        FLATBUFFERS_FALLTHROUGH();  // else fall thru
       default: return "Table";
     }
   }
@@ -327,7 +327,7 @@ class GeneralGenerator : public BaseGenerator {
       case BASE_TYPE_UINT: return Type(BASE_TYPE_LONG);
       case BASE_TYPE_VECTOR:
         if (vectorelem) return DestinationType(type.VectorType(), vectorelem);
-        // else fall thru
+        FLATBUFFERS_FALLTHROUGH(); // else fall thru
       default: return type;
     }
   }
@@ -371,7 +371,7 @@ class GeneralGenerator : public BaseGenerator {
       case BASE_TYPE_UINT: return " & 0xFFFFFFFFL";
       case BASE_TYPE_VECTOR:
         if (vectorelem) return DestinationMask(type.VectorType(), vectorelem);
-        // else fall thru
+        FLATBUFFERS_FALLTHROUGH(); // else fall thru
       default: return "";
     }
   }

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");

src/idl_parser.cpp

@@ -411,10 +411,8 @@ CheckedError Parser::Next() {
           }
           cursor_ += 2;
           break;
-        } else {
-          // fall thru
         }
-        FLATBUFFERS_ATTRIBUTE(fallthrough);
+        FLATBUFFERS_FALLTHROUGH(); // else fall thru
       default:
         const auto has_sign = (c == '+') || (c == '-');
         // '-'/'+' and following identifier - can be a predefined constant like:

src/reflection.cpp

@@ -431,7 +431,7 @@ Offset<const Table *> CopyTable(FlatBufferBuilder &fbb,
               break;
             }
           }
-          // FALL-THRU
+          FLATBUFFERS_FALLTHROUGH(); // fall thru
           default: {  // Scalars and structs.
             auto element_size = GetTypeSize(element_base_type);
             if (elemobjectdef && elemobjectdef->is_struct())
@@ -466,7 +466,7 @@ Offset<const Table *> CopyTable(FlatBufferBuilder &fbb,
           break;
         }
       }
-      // ELSE FALL-THRU
+      FLATBUFFERS_FALLTHROUGH(); // fall thru
       case reflection::Union:
       case reflection::String:
       case reflection::Vector: