[ASTImporter] Add support for importing ChooseExpr AST nodes.
This allows ASTs to be merged when they contain ChooseExpr (the GNU
builtin_choose_expr construction). This is needed, for example, for
cross-CTU analysis of C code that makes use of builtin_choose_expr.
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
Reviewers: shafik, a_sidorin, martong, aaron.ballman
Subscribers: aaron.ballman, rnkovacs, jdoerfert, cfe-commits
Differential Revision: https://reviews.llvm.org/D58292