[modules] Merge ObjC interface ivars with anonymous types.

Without the fix ivars with anonymous types can trigger errors like

> error: 'TestClass::structIvar' from module 'Target' is not present in definition of 'TestClass' provided earlier
> [...]
> note: declaration of 'structIvar' does not match

It happens because types of ivars from different modules are considered
to be different. And it is caused by not merging anonymous `TagDecl`
from different modules.

To fix that I've changed `serialization::needsAnonymousDeclarationNumber`
to handle anonymous `TagDecl` inside `ObjCInterfaceDecl`. But that's not
sufficient as C code inside `ObjCInterfaceDecl` doesn't use interface
decl as a decl context but switches to its parent (TranslationUnit in
most cases).  I'm changing that to make `ObjCContainerDecl` the lexical
decl context but keeping the semantic decl context intact.

Test "check-dup-decls-inside-objc.m" doesn't reflect a change in
functionality but captures the existing behavior to prevent regressions.

rdar://85563013

Differential Revision: https://reviews.llvm.org/D118525