[modules] Update visibility for merged ObjCInterfaceDecl definitions.

We keep using the first encountered definition and need to take into
account visibility from subsequent definitions. For example, if the
first definition is hidden and the second is visible, we need to make
the first one visible too.

rdar://82263843

Differential Revision: https://reviews.llvm.org/D110453

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -1181,6 +1181,7 @@ void ASTDeclReader::MergeDefinitionData(ObjCInterfaceDecl *D,
   if (DD.Definition != NewDD.Definition) {
     Reader.MergedDeclContexts.insert(
         std::make_pair(NewDD.Definition, DD.Definition));
+    Reader.mergeDefinitionVisibility(DD.Definition, NewDD.Definition);
   }
 
   // FIXME: odr checking?

clang/test/Modules/merge-objc-interface-visibility.m

@@ -0,0 +1,61 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: %clang_cc1 -emit-llvm -o %t/test.bc -F%t/Frameworks %t/test.m -DHIDDEN_FIRST=1 \
+// RUN:            -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/modules.cache
+// RUN: %clang_cc1 -emit-llvm -o %t/test.bc -F%t/Frameworks %t/test.m -DHIDDEN_FIRST=0 \
+// RUN:            -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/modules.cache
+
+// Test a case when Objective-C interface is imported both as hidden and as visible.
+
+//--- Frameworks/Foundation.framework/Headers/Foundation.h
+@interface NSObject
+@end
+
+//--- Frameworks/Foundation.framework/Modules/module.modulemap
+framework module Foundation {
+  header "Foundation.h"
+  export *
+}
+
+//--- Frameworks/Regular.framework/Headers/Regular.h
+#import <Foundation/Foundation.h>
+@interface Regular : NSObject
+@end
+
+//--- Frameworks/Regular.framework/Modules/module.modulemap
+framework module Regular {
+  header "Regular.h"
+  export *
+}
+
+//--- Frameworks/RegularHider.framework/Headers/Visible.h
+// Empty, file required to create a module.
+
+//--- Frameworks/RegularHider.framework/Headers/Hidden.h
+#import <Foundation/Foundation.h>
+@interface Regular : NSObject
+@end
+
+//--- Frameworks/RegularHider.framework/Modules/module.modulemap
+framework module RegularHider {
+  header "Visible.h"
+  export *
+
+  explicit module Hidden {
+    header "Hidden.h"
+    export *
+  }
+}
+
+//--- test.m
+
+#if HIDDEN_FIRST
+#import <RegularHider/Visible.h>
+#import <Regular/Regular.h>
+#else
+#import <Regular/Regular.h>
+#import <RegularHider/Visible.h>
+#endif
+
+@interface SubClass : Regular
+@end