Tweaks and fixes to cpp11-migrate generated tests
    
* Fixed a comment typo
* Changed 'autogen' to 'generated-tests'
* Made the clean target not fail if the 'generated-tests' directory
  doesn't already exist.


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@175624 91177308-0d34-0410-b5e6-96231b3b80d8