[ASTImporter] Add support for importing GenericSelectionExpr AST nodes.

This allows ASTs to be merged when they contain GenericSelectionExpr
nodes (this is _Generic from C11). This is needed, for example, for
CTU analysis of C code that makes use of _Generic, like the Linux
kernel.

The node is already supported in the AST, but it didn't have a matcher
in ASTMatchers. So, this change adds the matcher and adds support to
ASTImporter. Additionally, this change adds support for structural
equivalence of _Generic in the AST.

Reviewed By: martong, aaron.ballman

Differential Revision: https://reviews.llvm.org/D92600