Introduce clang_getCursorPrettyPrinted() for pretty printing
declarations. Expose also PrintingPolicy, so the user gets more
fine-grained control of the entities being printed.
The already existing clang_getCursorDisplayName() is pretty limited -
for example, it does not handle return types, parameter names or default
arguments for function declarations. Addressing these issues in
clang_getCursorDisplayName() would mean to duplicate existing code
(e.g. clang::DeclPrinter), so rather expose new API to access the