[OBJC] __attribute__((NSObject)) types should be usable as lightweight generic types

[dmaclach]

typedef __attribute__((NSObject)) struct Foo *FooRef;
NSArray<FooRef> *array;

Currently gives you the error type argument 'FooRef' (aka 'const struct Foo *') is neither an Objective-C object nor a block type
which is not true given that it has __attribute__((NSObject)) and as per https://clang.llvm.org/docs/AutomaticReferenceCounting.html#retainable-object-pointers is a retainable type.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: dmaclach (dmaclach)

``` typedef __attribute__((NSObject)) struct Foo *FooRef; NSArray<FooRef> *array; ```

Currently gives you the error type argument 'FooRef' (aka 'const struct Foo *') is neither an Objective-C object nor a block type
which is not true given that it has __attribute__((NSObject)) and as per https://clang.llvm.org/docs/AutomaticReferenceCounting.html#retainable-object-pointers is a retainable type.

[nico]

(dmac: If you say something like Fixes #84592 in your commit description, github will auto-close the issue when the commit gets merged.)