[llvm-dev] Add make_unqiue like function for IntrusiveRefCntPtr

[Nathan James via llvm-dev]

Hi all,

How would people feel about including a factory function for creating
IntrusiveRefCntPtr objects, akin to how std::make_unique and its
variants work. For the most part this change would just be code sugar,
however it will clean up definitions and remove the need to duplicate
the type name. I can also see this as a stepping stone to gradually
making the constructor `IntrusiveRefCntPtr(T*)` explicit, preventing
some accidental conversions which can easily result in undefined
behaviour if say the object lives on the stack or is already owned by
another object.

Take this which is an example seen often throught the code base,
appologies in advance if email formatting is kaput.
>IntrusiveRefCntPtr<OverlayFileSystem> OFS(
>    new OverlayFileSystem(getRealFileSystem()));
>IntrusiveRefCntPtr<InMemoryFileSystem> IMFS(new InMemoryFileSystem);
>OFS->pushOverlay(IMFS);

Using A factory these could be cleaned up a lot.
>auto OFS = makeIntrusive<OverlayFileSystem>(getRealFileSystem());
>auto IMFS = makeIntrusive<InMemoryFileSystem>();
>OFS->pushOverlay(IMFS);

This wouldn't be in violation of the rules on using `auto` for
declarations as the type is spelt in the initialisation.

Thanks for reading,
Nathan James