[clang-tidy] modernize-use-auto: don't remove stars by default

Summary:
By default, modernize-use-auto check will retain stars when replacing an explicit type with `auto`: `MyType *t = new MyType;` will be changed to `auto *t = new MyType;`, thus resulting in more consistency with the recommendations to use `auto *` for iterating over pointers in range-based for loops: http://llvm.org/docs/CodingStandards.html#beware-unnecessary-copies-with-auto

The new  `RemoveStars` option allows to revert to the old behavior: with the new option turned on the check will change `MyType *t = new MyType;` to `auto t = new MyType;`.

Reviewers: aaron.ballman, sbenza

Subscribers: Eugene.Zelenko, cfe-commits

Differential Revision: http://reviews.llvm.org/D20917

llvm-svn: 271739