Use LANGUAGE OBJCXX rather than COMPILE_FLAGS "-x objective-c++"

We sometimes use Objective-C++ code in files with a .cpp extension,
to avoid the churn of adding a foo_mac.mm file. Instead of manually
telling the compiler to build these files in Objective-C++ mode, we
use CMake's intended mechanism, which means genex constructs such as
$<$<COMPILE_LANGUAGE:OBJCXX> will work for these files as well.

Pick-to: 6.5 6.2
Change-Id: If295c3f34f6bee9f4d9f877f519c9c7770665fee
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

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