Common: Avoid std::function overhead in ScopeGuard

[leoetlino]

So far in all our uses of ScopeGuard, the type of the callable is
usually just a lambda or a function pointer, so there is no need
to rely on std::function's type erasure.

While the cost of using std::function is probably negligible, it still
causes some unnecessary overhead that can be avoided by making
ScopeGuard a templated class. Thanks to class template argument
deduction in C++17 most existing usages do not even need to be changed.

See https://godbolt.org/z/KcoPni for a comparison between
a ScopeGuard that uses std::function and one that doesn't

[CookiePLMonster]

Very good change, I had it in mind more than once in the past so glad to see it done. However, I am pretty sure m_finalizer doesn't need to be a std::optional and you can "null" it by assigning {} to it.

[leoetlino]

Unfortunately that doesn't work for lambdas.

../Source/Core/Common/ScopeGuard.h:24:9: error: could not convert ‘((Common::ScopeGuard<Core::EmuThread(std::unique_ptr<BootParameters>, WindowSystemInfo)::<lambda()> >*)this)->Common::ScopeGuard<Core::EmuThread(std::unique_ptr<BootParameters>, WindowSystemInfo)::<lambda()> >::m_finalizer’ from ‘Core::EmuThread(std::unique_ptr<BootParameters>, WindowSystemInfo)::<lambda()>’ to ‘bool’
   24 |     if (m_finalizer)
      |         ^~~~~~~~~~~
      |         |
      |         Core::EmuThread(std::unique_ptr<BootParameters>, WindowSystemInfo)::<lambda()>

...

../Source/Core/Common/ScopeGuard.h:27:19: error: use of deleted function ‘Core::EmuThread(std::unique_ptr<BootParameters>, WindowSystemInfo)::<lambda()>& Core::EmuThread(std::unique_ptr<BootParameters>, WindowSystemInfo)::<lambda()>::operator=(const Core::EmuThread(std::unique_ptr<BootParameters>, WindowSystemInfo)::<lambda()>&)’
   27 |       m_finalizer = {};
      |       ~~~~~~~~~~~~^~~~
../Source/Core/Core/Core.cpp:524:71: note: a lambda closure type has a deleted copy assignment operator
  524 |   Common::ScopeGuard controller_guard{[init_controllers, init_wiimotes] {

[CookiePLMonster]

Ah, that's fine then. Your godbolt example also shows that optimizers seem pretty good at realizing std::optional can be skipped, and it provides some much needed clarity here.