[TableGen] Give meaningful msg for def use in multiclass

When one mistakenly specifies 'def' instead of using 'defm',
the error message is quite misleading: 'Couldn't find class..'
Instead, it should recommend using defm if the multiclass of
same name exists.

Reviewed By: hfinkel

Differential Revision: https://reviews.llvm.org/D59294 

llvm-svn: 356985