Initial support for types_fwd.h headers

[antonl2]

Summary: This draft is doing very basic integration:

• types_fwd.h is generated for each cpp2 enabled thrift_library
• Only fwd decls of structs are in types_fwd.h
• types_fwd.h is part of existing cpp2-types library

Differential Revision: D46574522

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D46574522

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D46574522

[facebook-github-bot]

This pull request has been merged in facebook/hhvm@95ed44b7a84591ad978875041fa32361402e4dd9.