swift234

[ClangImporter] Preserve macros from all implicit submodules. (#3983)

...instead of picking one definition arbitrarily. This comes from the
new "lookup table" design in Swift 3---we no longer just look for any
"visible" (imported) macro definition, but instead need to know them
up front. This works fine when there's only one definition per module,
but for modules like 'OpenGL' on macOS, with mutually-exclusive
submodules 'GL' and 'GL3', the compiler was arbitrarily deciding that
all of the macros the submodules had in common belonged to 'GL'.

The new model tries to decide if it's possible for two modules to be
imported separately, and keeps both macro entries if possible, only
deduplicating equivalent definitions at the last minute (when
importing into Swift). This /still/ doesn't perfectly match the
behavior you'd get in C, where a submodule and its parent module could
theoretically have conflicting definitions and you'd be fine as long
as you only imported one of them, but hopefully (a) it's close enough,
and (b) nobody is doing that. (The Swift compiler will prefer the
definition in the parent module even if the submodule is the only one
imported.)

rdar://problem/26731529