[NFCI]Create CommonAttributeInfo Type as base type of *Attr and ParsedAttr.

In order to enable future improvements to our attribute diagnostics,
this moves info from ParsedAttr into CommonAttributeInfo, then makes
this type the base of the *Attr and ParsedAttr types. Quite a bit of
refactoring took place, including removing a bunch of redundant Spelling
Index propogation.

Differential Revision: https://reviews.llvm.org/D67368

llvm-svn: 371875