PR7256: Provide a fixit for incorrect destructor declarations

Fix by Ismail Pazarbasi (ismail.pazarbasi@gmail.com), review by Dmitri Gribenko.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177546 91177308-0d34-0410-b5e6-96231b3b80d8