SE-0460: Introduce @specialized attribute

[aschwaighofer]

Implements SE-0460 -- the non-underscored version of @specialized.

It allows to specify "internal" (not abi affecting) specializations.

Quoting from the proposal:

A new attribute, @specialized, will allow the author of a function to cause the compiler to generate specializations of that function. In the body of the unspecialized version, the types are first checked to see if they are of one of the specialized types. If they are, the specialized version will be called.

So in our example above:

extension Sequence where Element: BinaryInteger {
  @specialized(where Self == [Int])
  func sum() -> Double {
    reduce(0) { $0 + Double($1) }
  }
}

A specialized version of [Int].sum will be generated in the same way as if it had been specialized for a callsite. And inside the unspecialized generic code, the additional check-and-redispatch logic will be inserted at the start of the function.

rdar://150033316

[aschwaighofer]

test with swiftlang/swift-syntax#3082

@swift-ci test

[aschwaighofer]

@swift-ci test

[aschwaighofer]

test with swiftlang/swift-syntax#3082

@swift-ci test

[aschwaighofer]

test with swiftlang/swift-syntax#3082

@swift-ci test

[aschwaighofer]

test with swiftlang/swift-syntax#3082

@swift-ci test