[Typechecker] Fix a few regressions with @autoclosure

[theblixguy]

Since Swift 5.1, we have a few regressions around the use of @autoclosure.

1. We are crashing if the @autoclosure type is not a function type, but just a variadic type. For example:

func foo(_ closure: @autoclosure String...) {} // crash

2. We're not correctly enforcing the use of @autoclosure in parameter position. For example, the following does not get diagnosed, even though it should:

let foo: Array<@autoclosure String> = [] // no error

3. If we have () -> String... as the type and we pass it a value, then we crash in CSApply after diagnosing the error. I have filed a separate bug for it.

Resolves SR-11934, SR-11938
Resolves: rdar://problem/57824033

[theblixguy]

@swift-ci please smoke test

[theblixguy]

@swift-ci please smoke test macOS

[xedin]

@theblixguy Could we please limit this to your original fix? I can get that into master and 5.2. Requestifying can happen after.

[theblixguy]

Sure! I’ll split the request into a separate PR.

[xedin]

Thank you!

[theblixguy]

@xedin Done. I have kept the core logic as is, but removed all the requestification bits. It passes all tests in attr_autoclosure.swift.

[xedin]

@swift-ci please test

[xedin]

@swift-ci please test source compatibility

[theblixguy]

Okay, tests have pretty much passed (just waiting for Release source compat). I'll create a new PR for 5.2 branch

[xedin]

Sounds good! Thank you, @theblixguy!