Add and use a couple helper functions for issuing policy warnings. This
improves consistency and allows some simplification of many call sites.
(One or two instances in particular are greatly simplified.)
Add new diagnostic categories CMD_POLICY and CMD_EXPERIMENTAL, both as
children of CMD_AUTHOR. Use the former when issuing policy warnings. Use
the latter when issuing experimental feature warnings.
Find places that are currently relying on diagnostic-specific message
types to issue diagnostics and replace these with calls to the new
diagnostic methods.
Move AddHeaderSetVerification, GenerateHeaderSetVerificationFile,
ResolveHeaderLanguage, and GenerateStubForLanguage out of the
monolithic cmGeneratorTarget.cxx into a new split file following
the existing cmGeneratorTarget_*.cxx pattern.
Matthew Woehlke
Marc Chevrier
Mickaël Germain