libclang: split out the documentation comment API

It's possible that the "comment AST" may be replaced or split out in the
midterm, any anyway this makes the headers easier to read.

Developers don't currently need to include "clang-c/Documentation.h" explicitly
and there's no macro to test for availability yet.

The raw comment and brief comment accessors have been kept in Index.h though
brief support may also move here as a separate proposal.

This is not a deprecation, just a gentle separation of concerns as we look to
simplify the built-in representation of comment nodes and support external
comment processors.