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.

Author: tshortli

lib/ClangImporter/ImportDecl.cpp

@@ -7178,7 +7178,7 @@ void SwiftDeclConverter::recordObjCOverride(AbstractFunctionDecl *decl) {
   // Dig out the Objective-C superclass.
   SmallVector<ValueDecl *, 4> results;
   superDecl->lookupQualified(superDecl, DeclNameRef(decl->getName()),
-                             decl->getLoc(), NL_QualifiedDefault,
+                             decl->getLoc(), NL_QualifiedDefault | NL_IgnoreMissingImports,
                              results);
   for (auto member : results) {
     if (member->getKind() != decl->getKind() ||

test/NameLookup/Inputs/MemberImportVisibility/Bridging.h

@@ -1,9 +1,18 @@
 @import Categories_A;
 
+@interface NSObject (BridgingHeader)
+- (void)overridesCategoryMethodOnNSObject;
+@end
+
 @interface X (BridgingHeader)
 - (void)fromBridgingHeader;
+- (void)overridesCategoryMethodOnNSObject;
 @end
 
 struct StructInBridgingHeader {
   int member;
 };
+
+@interface ObjectInBridgingHeader : NSObject
+- (void)overridesCategoryMethodOnNSObject;
+@end

test/NameLookup/members_transitive_objc.swift

@@ -24,6 +24,7 @@ func test(x: X) {
   x.fromOverlayForC() // expected-member-visibility-error {{instance method 'fromOverlayForC()' is not available due to missing import of defining module 'Categories_C'}}
   x.fromSubmoduleOfD() // expected-member-visibility-error {{instance method 'fromSubmoduleOfD()' is not available due to missing import of defining module 'Categories_D'}}
   x.fromBridgingHeader()
+  x.overridesCategoryMethodOnNSObject()
 }
 
 func testAnyObject(a: AnyObject) {
@@ -37,6 +38,7 @@ func testAnyObject(a: AnyObject) {
   a.fromC() // expected-error {{value of type 'AnyObject' has no member 'fromC'}}
   a.fromOverlayForCObjC() // expected-error {{value of type 'AnyObject' has no member 'fromOverlayForCObjC'}}
   a.fromBridgingHeader()
+  a.overridesCategoryMethodOnNSObject()
 }
 
 extension StructInBridgingHeader {
@@ -48,3 +50,9 @@ extension StructInBridgingHeader {
     return member
   }
 }
+
+extension ObjectInBridgingHeader {
+  func test() {
+    overridesCategoryMethodOnNSObject()
+  }
+}