r301815 - Adapt to LLVM's rename of WeakVH to WeakTrackingVH; NFC

[Sanjoy Das via cfe-commits]

Hi Yaron,

On Wed, May 3, 2017 at 2:11 AM, Yaron Keren <yaron.keren at gmail.com> wrote:
> out-of-tree code that uses WeakVH will continue to compile OK but may not
> perform OK with WeakVH not tracking anymore.
> May be better not to reuse WeakVH yet so such code will fail compilation.

Typically we tend to focus on simplicity of the C++ API over its
backward compatibility, so I'd rather not choose a poorer name for
WeakVH solely for that reason.  Downstream users tracking ToT are
expected to proactively track upstream commits (as far as I know) for
this same reason.

However, I will make sure that the change shows up in the release
notes.  I've already sent out a heads up email to llvm-dev.

-- Sanjoy