Complex SFINAE expressions appear to be broken on the latest release of MSVC 2017.

MSVC++ 14.15 (Visual Studio 2017 version 15.8):
The parent change compiles fine.

MSVC++ 14.16 (Visual Studio 2017 version 15.9):
 `error C2039: 'type': is not a member of 'std'` for some CToReactorPtr specializations that use `std::enable_if`.

Replacing these `typename std::enable_if<T>::type` with `enable_if_t<T>` works around the issue (while also making the template expressions easier to read).

I was unable to find an existing MSVC bug report for this problem, and so I will create one.

Bug: swiftshader:141
Change-Id: I7e953dbc719d7cb656aa7f16656c3f86ffad5522
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38688
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>