Forward declare Objective-C classes as class, not typedef objc_object

Forward declaring an Objective-C class in Objective-C/C++ mode is done
by using the `@class` syntax, e.g.:

  @class NSString;

In C/C++ mode however there's no documented approach, so we chose
to flatten the type down to the opaque objc_object "base class":

  typedef struct objc_object NSString;

As it turns out, when Objective-C classes are used as arguments or return
types in C++, the signature they produce is equal to what it would have
been if the type was a normal class. For example:

  void foo(NSString *) -> __Z3fooP8NSString

The is due to @class in Objective-C++ just being just sugar, so an NSString
pointer is not treated as `struct objc_object *` but rather a pointer to a
distinct type, which then gets mangled as such by LLVM's Itanium mangler
in CXXNameMangler::mangleType(const ObjCObjectType *T).

With our current forward declaration however, we are expecting:

  void foo(NSString *) -> __Z3fooP11objc_object

As a consequence exported helper functions such as QString::fromNSString()
are not possible to use from plain C++ right now, as it will give a linker
error for the missing QString::fromNSString(objc_object*) function.

And even if we did define the extra signature, it would not be possible
to declare overloaded functions taking Objective-C classes, as they would
all produce ambiguous overloads in C++ mode.

To fix this we change the forward declaration to a plain old class,
which matches the signature in both Objective-C++ and plain C++ mode,
and allows overloads. This is a binary compatible change, as no client
were using any of these functions from C++ anyways as they would have
produced linker errors. It does have a slight source compatible break,
for clients that manually forward declared classes using the old style,
but that use-case is deemed fringe enough to accept, and clients can
work around this by defining Q_FORWARD_DECLARE_OBJC_CLASS to their
preferred format, which Qt will respect.

Pick-to: 6.2
Change-Id: I04813c60a7da22379dd9de1be56cc12c53a38232
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/global/qglobal.h

@@ -859,7 +859,7 @@ constexpr inline QTypeTraits::Promoted<T, U> qBound(const U &min, const T &val,
 #  ifdef __OBJC__
 #    define Q_FORWARD_DECLARE_OBJC_CLASS(classname) @class classname
 #  else
-#    define Q_FORWARD_DECLARE_OBJC_CLASS(classname) typedef struct objc_object classname
+#    define Q_FORWARD_DECLARE_OBJC_CLASS(classname) class classname
 #  endif
 #endif
 #ifndef Q_FORWARD_DECLARE_CF_TYPE