Change the error when a '+=' follows a declaration to suggest a fixit to '=' instead...

Change the error when a '+=' follows a declaration to suggest a fixit to '=' instead of just suggesting a ';'.

Old error:
plusequaldeclare1.cc:3:8: error: expected ';' at end of declaration
  int x += 6;
       ^
       ;

New error:
plusequaldeclare1.cc:3:9: error: invalid '+=' at end of declaration; did you
      mean '='?
  int x += 6;
        ^~
        =

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@148433 91177308-0d34-0410-b5e6-96231b3b80d8