Look for the C-Language Copy of UIApplicationMain

A long while back we added a copy of UIApplicaitonMain to the UIKit
overlay. This was intended to fix the signature of UIApplicationMain by
forcing clients to choose a non-deprecated copy with revised nullability
annotations that were compatible with the (then) interface of the (then)
CommandLine stdlib type. Since then, we've gone in a completely different
direction with type-based entrypoints and @main, but the compatibility
shims remain. This code was assuming that it would always find one
declaration of UIApplicationMain in the clang module, but that's not
a sound assumption to make. In certain scenarios, we can wind up finding
the overlay's copy of UIApplicationMain too. When that's the Swift
function, we reference a native-to-foreign thunk that we don't actually
wind up generating in the open-coding and this results in a nasty linker
error. Filter the available candidates here looking for the C copy of
UIApplicationMain.

rdar://101932338

lib/SILGen/SILGenFunction.cpp

@@ -631,16 +631,22 @@ void SILGenFunction::emitArtificialTopLevel(Decl *mainDecl) {
       ->loadModule(SourceLoc(),
                    ImportPath::Module(llvm::makeArrayRef(UIKitName)));
     assert(UIKit && "couldn't find UIKit objc module?!");
-    SmallVector<ValueDecl *, 1> results;
+    SmallVector<ValueDecl *, 2> results;
     UIKit->lookupQualified(UIKit,
                            DeclNameRef(ctx.getIdentifier("UIApplicationMain")),
                            NL_QualifiedDefault,
                            results);
-    assert(results.size() == 1
-           && "couldn't find a unique UIApplicationMain in the UIKit ObjC "
-              "module?!");
 
-    ValueDecl *UIApplicationMainDecl = results.front();
+    // As the comment above alludes, using a qualified lookup into UIKit is
+    // *not* sound. In particular, it's possible for the lookup to find the
+    // (deprecated) Swift copy of UIApplicationMain in UIKit and try to call
+    // that instead of the C entrypoint. Let's try to force this to happen.
+    auto FoundUIApplicationMain = llvm::find_if(results, [](const ValueDecl *VD) {
+      return !VD->getClangNode().isNull();
+    });
+    assert(FoundUIApplicationMain != results.end() &&
+           "Could not find a UIApplicationMain to call!");
+    ValueDecl *UIApplicationMainDecl = *FoundUIApplicationMain;
 
     auto mainRef = SILDeclRef(UIApplicationMainDecl).asForeign();
     SILGenFunctionBuilder builder(SGM);