Tweak fallthrough comments to get recognized by gcc7 (#4298)

GCC gained a new warning, -Wimplicit-fallthrough, which warns about
implicitly falling through a case statement. The regular expressions
used at the default level (-Wimplicit-fallthrough=3) don't match with
a colon at the end. The comment also needs to be followed (after
optional whitespace and other comments) by a 'case' or 'default'
keyword, i.e. it will not be recognized with a '}' between the comment
and the keyword.

src/reflection.cpp

@@ -420,8 +420,8 @@ Offset<const Table *> CopyTable(FlatBufferBuilder &fbb,
               offset = fbb.CreateVector(elements).o;
               break;
             }
-            // FALL-THRU:
           }
+          // FALL-THRU
           default: {  // Scalars and structs.
             auto element_size = GetTypeSize(element_base_type);
             if (elemobjectdef && elemobjectdef->is_struct())
@@ -458,8 +458,8 @@ Offset<const Table *> CopyTable(FlatBufferBuilder &fbb,
                      subobjectdef.bytesize());
           break;
         }
-        // else: FALL-THRU:
       }
+      // ELSE FALL-THRU
       case reflection::Union:
       case reflection::String:
       case reflection::Vector: