[clang-tidy] Add FixIt for swapping arguments in string-constructor-checker.

Summary:
Arguments can be swapped using fixit when they are not in macros.
This is the same implementation than SwappedArguments. Some code 
got lifted to be reused.

Others checks are not safe to be fixed as they tend to be bugs or errors.
It is better to let the user manually review them.

Reviewers: alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D19547

llvm-svn: 269208