[SE-0025] Adapt accessibility for fileprivate

[modocache]

The standard migration path for SE-0025 is to find-and-replace all uses of private with fileprivate. However, this expands accessibility beyond the necessary minimum. This commit uses compiler errors to determine the minimum set of changes.

This should only be tested/merged once apple/swift#3000 has landed in apple/swift. Thanks to @CodaFi for making it easy to enable the new fileprivate errors -- to discover the necessary changes I applied the following change to apple/swift#3000 (since that pull request now aliases private to fileprivate for migration purposes):

diff --git a/lib/AST/NameLookup.cpp b/lib/AST/NameLookup.cpp
index bee6b2f..149c8a4 100644
--- a/lib/AST/NameLookup.cpp
+++ b/lib/AST/NameLookup.cpp
@@ -1017,7 +1017,7 @@ static bool checkAccessibility(const DeclContext *useDC,

   switch (access) {
   case Accessibility::Private:
-//      return useDC == sourceDC || useDC->isChildContextOf(sourceDC);
+     return useDC == sourceDC || useDC->isChildContextOf(sourceDC);
   case Accessibility::FilePrivate:
     return useDC->getModuleScopeContext() == sourceDC->getModuleScopeContext();
   case Accessibility::Internal: {

We could probably refactor a bit after this pull request, to reduce API from fileprivate to private where possible.

[modocache]

apple/swift#3391 was merged, adding a fileprivate alias that acts identically to private for now. I wonder how this patch works with that alias...

@swift-ci please test

[mike-ferris]

Jordan submitted a new request that has less diffs (just one, actually) because the proposal was amended such that not all the conversions in this patch are actually needed anymore.

[modocache]

Sounds good; closing this in favor of #134. Thanks @jrose-apple!