[ASTExporter] Support CXXNamedCastExpr

Summary:
[ASTExporter] Support CXXNamedCastExpr

This is an example diff based on the code discussed with @irp.

One important difference: I decided to follow the class hierarchy of clang more precisely than the original code of ASTDumper.
More precisely:
- VisitCXXNamedCastExpr calls VisitCastExpr
- VisitCastExpr takes care of the 'base path' and the 'cast kind' : this code benefits VisitCXXNamedCastExpr.

In a next iteration (e.g. to add CXXFunctionalCastExpr) we _might_ want to go further, that is, define VisitExplicitCastExpr and make VisitCXXNamedCastExpr call it instead of VisitCastExpr, to factorize 'qual_type'.

Test Plan:
make -C libtooling test
make -C clang-ocaml test

Reviewers: ddino, irp

Reviewed By: irp

Subscribers: irp

Differential Revision: https://phabricator.fb.com/D1410633

Tasks: 3964417