[AST] Pack GenericSelectionExpr

Store the controlling expression, the association expressions and the
corresponding TypeSourceInfos as trailing objects.

Additionally use the bit-fields of Stmt to store one SourceLocation,
saving one additional pointer. This saves 3 pointers in total per
GenericSelectionExpr.

Differential Revision: https://reviews.llvm.org/D57104

Reviewed By: aaron.ballman

Reviewers: aaron.ballman, steveire
llvm-svn: 352276