CppParser: remove unnecessary #includes

Use forward declarations to avoid the need to include lots of clang headers.

This drops the compilation time for CppParser from 17.8 seconds to 14.1 seconds
for me.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@iki.fi>
Merge pull request #258 from tomba/remove-includes

CppParser: remove unnecessary #includes