[Index] Don't index non-public imported declarations (#25865)

Apart from mildly speeding up indexing, this also keeps the compiler
from running into issues with implementation-only imports that may not
be present while we're trying to index.

rdar://problem/52083709

lib/AST/Decl.cpp

@@ -2961,7 +2961,7 @@ getAccessScopeForFormalAccess(const ValueDecl *VD,
     if (resultDC->isLocalContext() || access == AccessLevel::Private)
       return AccessScope(resultDC, /*private*/true);
 
-    if (auto enclosingNominal = dyn_cast<NominalTypeDecl>(resultDC)) {
+    if (auto enclosingNominal = dyn_cast<GenericTypeDecl>(resultDC)) {
       auto enclosingAccess =
           getAdjustedFormalAccess(enclosingNominal, useDC,
                                   treatUsableFromInlineAsPublic);