Add a fixit for attributes incorrectly placed prior to 'struct/class/enum' keyword.

Suggest moving the following erroneous attrib list (based on location)
[[]] struct X;  
to 
struct [[]] X;

Additionally, added a fixme for the current implementation that diagnoses misplaced attributes to consider using the newly introduced diagnostic (that I think is more user-friendly).

llvm-svn: 321449