Add support for inferring generic arguments of result builder attributes

[calda]

Swift evolution proposal: swiftlang/swift-evolution#3062

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This PR adds support for inferring generic arguments of result builder attributes from the return type of the attached declaration.

For example, this sample code was previously rejected with the error "reference to generic type 'ArrayBuilder' requires arguments in <...>", but it now compiles successfully by inferring it as ArrayBuilder<String>.

@resultBuilder
struct ArrayBuilder<Element> {
  static func buildBlock(_ elements: Element...) -> [Element] {
    elements
  }
}

@ArrayBuilder
var stringArray: [String] {
  "foo"
  "bar"
}