Revert "[ClangImporter] Don't expect unmodularized structs to appear in 2 modules"

This reverts commit c8c3f9b.

With `isThisDeclarationADemotedDefinition` flag we expect the same
struct to be available in multiple modules.

Author: vsapsai

test/ClangImporter/overlay.swift

@@ -17,6 +17,5 @@ let encoding: UInt = NSUTF8StringEncoding
 let viaTypedef: Redeclaration.NSPoint = AppKit.NSPoint(x: 0, y: 0)
 Redeclaration.NSStringToNSString(AppKit.NSStringToNSString("abc")) // expected-warning {{result of call to 'NSStringToNSString' is unused}}
 
-/// The following error should soon be introduced by https://github.com/apple/llvm-project/pull/3497.
-let viaStruct: Redeclaration.FooStruct1 = AppKit.FooStruct1() // expected-error * {{module 'AppKit' has no member named 'FooStruct1'}}
+let viaStruct: Redeclaration.FooStruct1 = AppKit.FooStruct1()
 let forwardDecl: Redeclaration.Tribool = AppKit.Tribool() // expected-error {{no type named 'Tribool' in module 'Redeclaration'}}