Common: Add ArrayRef and use it for GL::Context

[TellowKrinkle]

Description of Changes

• Adds LLVM's ArrayRef to Common
• Switches GL::Context's pointer + length usage to ArrayRef

Rationale behind Changes

• Can use C++ iteration now
• The length of the return of GL::Context::GetAllVersionsList is no longer part of its type

Suggested Testing Steps

Launch the emulator with a GL backend

[CookiePLMonster]

There is also std::span/gsl::span, from what I can see by briefly looking into this code they are equivalent? https://github.com/microsoft/GSL/blob/main/include/gsl/span

std::span is C++20 so that might be a dealbreaker, but I think gsl::span has an identical API so that'd make potential future migration simplistic.

EDIT: gsl::span also has bounds checks: https://github.com/microsoft/GSL/wiki/gsl::span-and-std::span

[TellowKrinkle]

Switched to gsl