RequiredPtr: Fix template conditional for MSVC

[akien-mga]

Fixes #114206.

I'm not familiar with this syntax but a quick search on the error yielded https://stackoverflow.com/a/71512930, and that seems to solve it for @isaaccp.

I have no idea why MSVC on our CI doesn't have a problem though.

[dsnopek]

The code change seems fine.

We use the same trick in a whole bunch of other places in this class, but not all of them. For example, if you look up at the constructors that take T_Other, we're not doing it there.

So, I'm pretty unclear on when it's required and when it isn't, and since I haven't tested on MSVC myself, there's the possibility that more of these will need to be changed?

[akien-mga]

Yeah I have no clue either why this is needed in some cases and apparently not all. We could defensively use typename U = T everywhere then and never use T directly, but someone who understand this stuff better than I do should probably look into why this indirection is needed and why MSVC is touchy only about some of them.

[Ivorforce]

LGTM
MSVC is flaky enough that we don't have to explain it. If clang and GCC are.happy, but msvc isn't, it usually means we're dealing with another obscure msvc bug. So i don't think we need to go out of our way to understand the exact cause.