[Clang importer] Import incomplete SWIFT_SHARED_REFERENCE types

Normally, Swift cannot import an incomplete type. However, when we are
importing a SWIFT_SHARED_REFERENCE type, we're always dealing with
pointers to the type, and there is no need for the underlying type to
be complete. This permits a common pattern in C libraries where the
actual underlying storage is opaque and all APIs traffic in the
pointer, e.g.,

    typedef struct MyTypeImpl *MyType;
    void MyTypeRetain(MyType ptr);
    void MyTypeRelease(MyType ptr);

to use SWIFT_SHARED_REFERENCE to import such types as foreign
references, rather than as OpaquePointer.

Fixes rdar://155970441.

lib/ClangImporter/SwiftDeclSynthesizer.cpp

@@ -2560,7 +2560,7 @@ llvm::SmallVector<clang::CXXMethodDecl *, 4>
 SwiftDeclSynthesizer::synthesizeStaticFactoryForCXXForeignRef(
     const clang::CXXRecordDecl *cxxRecordDecl) {
 
-  if (cxxRecordDecl->isAbstract())
+  if (!cxxRecordDecl->isCompleteDefinition() || cxxRecordDecl->isAbstract())
     return {};
 
   clang::ASTContext &clangCtx = cxxRecordDecl->getASTContext();