Add fixit hints for misplaced C++11 attributes around class specifiers.

Following r168626, in class declaration or definition, there are a combination of syntactic locations 
where C++11 attributes could appear, and among those the only valid location permitted by standard is
between class-key and class-name. So for those attributes appear at wrong locations, fixit is used to 
move them to expected location and we recover by applying them to the class specifier.
 

llvm-svn: 171757