[clang][ASTImporter] Generic attribute import handling (first step).

Import of Attr objects was incomplete in ASTImporter.
This change introduces support for a generic way of importing an attribute.
For an usage example import of the attribute AssertCapability is
added to ASTImporter.
Updating the old attribute import code and adding new attributes or extending
the generic functions (if needed) is future work.

Reviewed By: steakhal, martong

Differential Revision: https://reviews.llvm.org/D109608