Specify when directives are included

[k2d222]

Directives are diagnostic, enable and requires. What should we do with them when they are spread accross files?

• For the global diagnostic filters directive, we can probably apply the same principle as for const_asserts Specify when const_assert statements are included #66, i.e. include them when at least one declaration in the module is referenced from the root module (with static usage analysis)
• For the enable directive, we discussed at some point the following compromise:
  • Enable directives must be included in all modules that use a given extension.
  • The root module must repeat all enable directives used in referenced modules (via static usage analysis).
• For the requires directive, since they are optional (contrary to enable), I suggest that we don't require the root module to repeat them, but we include in the final output those written in referenced modules (via static usage analysis).