ClangImporter: Ignore missing imports in SwiftDeclConverter::recordObjCOverride().

`recordObjCOverride()` records semantic overrides for imported Obj-C methods.
Since these methods are imported from a different language, it doesn't make
sense to enforce Swift's member import visibility rules when performing lookups
to find overridden methods. Doing so caused the Constrain Solver to lack
important information needed to eliminate overloads, resulting in erroneous
ambiguities.

Resolves rdar://141636723.

lib/ClangImporter/ImportDecl.cpp

@@ -7226,9 +7226,9 @@ void SwiftDeclConverter::recordObjCOverride(AbstractFunctionDecl *decl) {
     return;
   // Dig out the Objective-C superclass.
   SmallVector<ValueDecl *, 4> results;
-  superDecl->lookupQualified(superDecl, DeclNameRef(decl->getName()),
-                             decl->getLoc(), NL_QualifiedDefault,
-                             results);
+  superDecl->lookupQualified(
+      superDecl, DeclNameRef(decl->getName()), decl->getLoc(),
+      NL_QualifiedDefault | NL_IgnoreMissingImports, results);
   for (auto member : results) {
     if (member->getKind() != decl->getKind() ||
         member->isInstanceMember() != decl->isInstanceMember() ||