Find a way to stack allocate closures

[yorickpeterse]

Description

In #783 we're introducing the ability to define stack allocated types capable of storing both heap and stack allocated types. We should find a way to extend such functionality to also work for closures.

Stack allocating closures poses a few challenges:

• fn types are currently assumed to be pointers, not stack allocated data
• stack allocated closures don't have a uniform layout, as their size is dictated by the variables they capture
• closures may assign captured variables new values, which won't work reliably if we copy the stack data similar to inline types

For closures to be stack allocated, we most likely need to treat them similarly as generic type arguments and generate specialized versions. This however is likely to require extensive changes to how we specialize types and methods, so I'm not sure about this yet.

Related issues

• Introduce support for user-defined, stack allocated value types #750
• Make enums and tuples stack allocated types #780
• Implement some form of escape analysis to allocate values on the stack #776

Related work

• https://mjtsai.com/blog/2019/01/17/stack-allocation-for-non-escaping-swift-closures/
• [SR-7286] non-escaping closures aren't stack allocated swiftlang/swift#49834

[yorickpeterse]

For closures it might make more sense to keep their memory layout the same, but use escape analysis to allocate them on the stack. Most closures aren't likely to outlive the method they're defined in, so this might be good enough. This also removes the need for complex specialization changes.