Issue Description:

In generated Swift code, I use the #sourceLocation directive to map source lines to the corresponding location in my input files. Because the mapping may not exactly correspond to the syntactic structure of Swift, I expect to be able to use the directive at any position in Swift code.

In Swift version 4.0 (swiftlang-900.0.43 clang-900.0.22.8), the compiler produces an error when a #sourceLocation directive appears before the closing brace of an extension, struct, class, enum, or protocol declaration.

For example, the following code used to compile in Swift 3.2:

extension Int {
  var isEven: Bool {
    return self & 1 == 0
  }
  #sourceLocation(file: "Foobar.swift", line: 1)
}

The Swift 4 compiler, however, produces the following diagnostic:

Foobar.swift:1:1: error: expected declaration
}
^
test.swift:1:1: note: in extension of 'Int'
extension Int {
^

I can work around the error by adding a dummy member after the #sourceLocation directive, but that is not always a viable option in generated code.