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.
llvm-svn: 207392