Improve diagnostic on C++11 attribute specifiers that appear at wrong...

Improve diagnostic on C++11 attribute specifiers that appear at wrong syntactic locations around class specifiers.

This change list implemented logic that explicitly detects several combinations of locations where C++11 attribute
specifiers might be incorrectly placed within a class specifier. Previously we emit generic diagnostics like 
"expected identifier" for such cases; now we emit specific diagnostic against the misplaced attributes, this also 
fixed a bug in old code where attributes appear at legitimate locations were incorrectly rejected.

Thanks to Richard Smith for reviewing!

llvm-svn: 168626