Fix the Effective Access Level of package Declarations

swiftc mis-compiles package declarations today because it reports the effective access level of package declarations as less than public. There are a bunch of places in the optimizer that are checking the effective access level against an upper bound of public, so a lot of code winds up internalized or optimized as though it were internal when it definitely is not.

Fixes rdar://118081829

Author: Robert Widmann

lib/AST/Decl.cpp

@@ -4120,6 +4120,15 @@ AccessLevel ValueDecl::getEffectiveAccess() const {
   case AccessLevel::Open:
     break;
   case AccessLevel::Package:
+    if (getModuleContext()->isTestingEnabled() ||
+        getModuleContext()->arePrivateImportsEnabled()) {
+        effectiveAccess = getMaximallyOpenAccessFor(this);
+    } else {
+        // Package declarations are effectively public within their
+        // package unit.
+        effectiveAccess = AccessLevel::Public;
+    }
+    break;
   case AccessLevel::Public:
   case AccessLevel::Internal:
     if (getModuleContext()->isTestingEnabled() ||